Future of Third Rail

[zwk500]

If you where going to change from 3rd rail to AC OHLE eventually, could it be done in stages over 10 - 15 years as follows:

1) Install DC OHLE
2) Leave DC OHLR running for 5 years
3) Replace DC OHLE with AC OHLE

The issue I see that, is as I have stated before you will need a locomotive or train to be able to operate AC OHLE, DC OHLE and third rail unless you use as others have stated Electric/diesel engine or train that can be later converted to be electric/battery.

No, you would not out up a completely different spec OLE for 5 years. You'd go from 3rd rail to AC OLE direct.

 

[AM9]

No, you would not out up a completely different spec OLE for 5 years. You'd go from 3rd rail to AC OLE direct.

It would be ludicrous inventing, (and implementing), an intermediate standard just to allow the conversion to be dragged out over such a short time, especially when about one third of the 3rd rail stock is already capable of running under OLE. In addition, much of it has run regularly on both systems within single services.

 

[A0]

How is this managed at places like Mitre Bridge and where the North London Line changes between 25Kv and DC (Willesden isnt it?), and Drayton Park, and on Thameslink?

In the case of Thameslink, with lots of very expensive equipment which @Bald Rick has explained on other posts when this question has been asked.

 

[gg1]

In the case of Thameslink, with lots of very expensive equipment which @Bald Rick has explained on other posts when this question has been asked.

Would battery tech help significantly in this regard, ie having a very short (no more than a kilometre) non-energised section between the OHLE and 3rd rail elements of a route so a train would change from AC>Battery>DC?

For such a short distance you would only need a small battery, no need for a full blown BEMU.

 

[WAO]

If there were a sincere, valid objection to 3rd rail then there would be a program, even if slow, of replacement, rather than a bottom-covering objection to small extensions and a mis-application of electrical safety rules (which previously exempted railways).

However I agree with a PW charge-hand friend (and his wife) that DC is less than desirable and suggest that, even if there is completion of the DC network (North Downs, Uckfield, etc), a start is made on replacement.

The two top projects for me would be AC from Pirbright Junction to Weymouth, as this is a fast route that also carries freight and is demanding of a DC system. My second would be Merseyrail's tiny 76 route miles. This tends to isolate suburban Liverpool/Merseyside at present, a state of affairs inimical to a trading region. The tunnel problem would be solved either by the new battery stock or by use of the ROCS system, which might just fit in the 4.7m dia tunnels.

Some chance!

WAO

 

[mr_jrt]

Presumably at the very least you could install all the AC gantries over an operating 3rd rail installation, and ideally, the wire runs as well. I would imagine the actual switchover would be comparatively short, and mainly consist of the track-level changes (isolating the 3rd rail, the earthing, etc). Rather assumes all the other prequisites like clearances and signal sighting gets dealt with beforehand, though.

RE: Liverpool, I'd personally be tempted to leave the 3rd rail in the tunnels, but given the units are going to have batteries regardless, then maybe that is indeed a good opportunity to get rid of that island. Out of curiosity, how have Liverpool avoided the issues London had with corrosion which lead to the 4 rail system used there? Would batteries have any benefit to that end?

 

[WAO]

Out of curiosity, how have Liverpool avoided the issues London had with corrosion which lead to the 4 rail system used there? Would batteries have any benefit to that end?

The Link and Loop tunnels were relatively short and mostly concrete lined with CI segments used in places (such as Paradise St Junction). They are also in red sandstone not London Clay but they are very wet and corrosion of track fittings has been a problem. Groundwater ingress is very great and it has been used for condenser cooling of refrigeration systems, something of which LUL is now aware. While batteries would solve some problems, carrying around a large energy store (like a tank of diesel), under a passenger train, must raise the issue of fire safety in a confined tunnel.

WAO

 

[HSTEd]

Would anyone happen to know what the typical conductor rail to earth resistance is on third and fourth rail schemes?

I was just wondering if you could do something with an RCD for four rail schemes, but that would depend on the fault current through a person being significant compared to normal leakage. Although LU style voltage biasing is designed to keep leakage from both rails approximately equal in magnitude.

EDIT:

BS EN 50122-1 'Railway applications. Fixed installations. Electrical safety, earthing and the return circuit' includes electrical clearances for overhead conductor line systems at various voltages.
Electrical clearance is '>0mm' for low voltage, 30mm for 3kV, 150mm for 15kV and 240mm for 25kV.

This clearance is apparently driven by the need to resist switching overvoltage transients as large as 125kV in a 25kV system.

EDIT #2:

The minimum permanent voltage permitted between rails on the LU 630V four-rail system is ~400V. With the standard 2:1 biasing that would be something like +270V/-135V. If you accept unbalanced current leakage [with the RCDs shimmed appropriately] you could force it to +200V/-200V, both are within the range where RCD disconnection would be potentially acceptable (according to BS EN 50122-1), albeit it would have to be significantly more rapid in the 2:1 case. Alternatively you could go to ~+270V/-270V and feed at 540V.

Obviously feeding the trains at 400-540V is not ideal, but if you had superconducting feeder cables it might be workable. Could be somewhat useful in situations where you want to extend the London Underground without having to resort to trying to build overhead wire-capable tube stock. For example, getting the Met to Aylesbury to dispense with the whole operational headache of the current arrangement.

Rather a crude, brute-force, solution though.

 

[Pigeon]

Would anyone happen to know what the typical conductor rail to earth resistance is on third and fourth rail schemes?

The figure I've encountered for the LU ground resistances is 10k, but it's never made clear which rail that refers to, ie. whether it's 10k for the 420V rail and 5k for the 210V one, or 10k for the 210V one and 20k for the 420V one.

Third rail is of course "assume zero"

Rather a crude, brute-force, solution though.

Solution to what? I fail to see what it's intended to achieve.

 

[HSTEd]

Solution to what? I fail to see what it's intended to achieve.

Allows a fourth rail train to travel to places that are not currently electrified with third/fourth rail by overcoming the ORR, de-facto, prohibition on new installations. If touch voltage limits are complied with then rationale for that position is eliminated.

 

[Bald Rick]

How is this managed at places like Mitre Bridge and where the North London Line changes between 25Kv and DC (Willesden isnt it?), and Drayton Park, and on Thameslink?

In the case of Thameslink, with lots of very expensive equipment which @Bald Rick has explained on other posts when this question has been asked.

The issue is where you have a high DC current draw in an AC area. This is a very big issue in the TL core, as the DC system there isn’t just Thameslink, but it’s very close (and connected to for feeding / return purposes) all the lines out of Charing X and Cannon St.

This isn’t a problem at Acton, or Mitre Bridge, or Drayton Park.

It very much would be a problem at, say, Basingstoke.

The complicated (= expensive) kit at Blackfriars / City TL is also not perfect, which is why the signalling insulated rail joints burn out very year or so.

 

[WAO]

I've sometimes wondered whether in mixed ac/dc areas, the dc sub-station nearest should be fed from the 25kV supply, so that everything comes off the one transformer (and phase). It would mean a lower transformer tapping than for the 33kV but most units have -15%.

Any mileage?

WAO

 

[Pigeon]

Allows a fourth rail train to travel to places that are not currently electrified with third/fourth rail by overcoming the ORR, de-facto, prohibition on new installations. If touch voltage limits are complied with then rationale for that position is eliminated.

Ah, I see.

I believe the value of the balance resistors is what it is at least partly to limit the maximum rail-to-ground fault current to a "safe" value (10k @ 210V = 21mA). However the system is also designed to be "lenient" towards rail-to-ground faults; they are an "attention required here" indicator rather than a "blow the breakers" trigger as on third rail. It will quite happily keep going with one rail shorted to ground and the other pushed up to 630V, and in that condition of course the rail-to-ground fault current available from that other rail is very large.

To what extent that would have to be re-thought to perpetrate a successful regulatory weaselism I am not prepared to speculate

 

[MarkyT]

Out of curiosity, how have Liverpool avoided the issues London had with corrosion which lead to the 4 rail system used there? Would batteries have any benefit to that end?

Another UK railway with 3rd rail in a largely iron segment tunnel network is the Glasgow subway, although it was only converted to electric traction from cable haulage in the late 1930s. I'm not convinced that stray return currents are necessarily much of a problem in DC systems as long as good modern insulation pads etc provided between running rails and bearers. That is the current state of the art for modern DC tramways to avoid stray current escaping into utility pipes and other surrounding metalwork, staying predominantly in the running rails which are often embedded in a continuous narrow trough in roadway sections backfilled with an insulating material. I have a theory that London's early decision to opt for a four rail power system was as much concerned with isolating traction current from the track circuits of signal systems, which in 3rd rail installations require expensive high current impedance bonds to separate traction and signalling current and which under certain fault conditions can result in high traction currents being injected into signalling equipment, often with fiery or explosive results, which would be very dangerous indeed underground. Metro systems were among the first adopters of continuous track circuiting and colour light signals.

 

[Ken H]

Another UK railway with 3rd rail in a largely iron segment tunnel network is the Glasgow subway, although it was only converted to electric traction from cable haulage in the late 1930s. I'm not convinced that stray return currents are necessarily much of a problem in DC systems as long as good modern insulation pads etc provided between running rails and bearers. That is the current state of the art for modern DC tramways to avoid stray current escaping into utility pipes and other surrounding metalwork, staying predominantly in the running rails which are often embedded in a continuous narrow trough in roadway sections backfilled with an insulating material. I have a theory that London's early decision to opt for a four rail power system was as much concerned with isolating traction current from the track circuits of signal systems, which in 3rd rail installations require expensive high current impedance bonds to separate traction and signalling current and which under certain fault conditions can result in high traction currents being injected into signalling equipment, often with fiery or explosive results, which would be very dangerous indeed underground. Metro systems were among the first adopters of continuous track circuiting and colour light signals.

Wasnt there one tube line that had the rails in reverse polarity compared to the rest of the system? Think the Central London Railway. Something to do with earth leakage to the cast iron tunnel lining. The Central Line tunnels are smaller than the Yerkes tubes.
Apologies if I have misremembered.

 

[RobShipway]

The issue is where you have a high DC current draw in an AC area. This is a very big issue in the TL core, as the DC system there isn’t just Thameslink, but it’s very close (and connected to for feeding / return purposes) all the lines out of Charing X and Cannon St.

This isn’t a problem at Acton, or Mitre Bridge, or Drayton Park.

It very much would be a problem at, say, Basingstoke.

The complicated (= expensive) kit at Blackfriars / City TL is also not perfect, which is why the signalling insulated rail joints burn out very year or so.

Another UK railway with 3rd rail in a largely iron segment tunnel network is the Glasgow subway, although it was only converted to electric traction from cable haulage in the late 1930s. I'm not convinced that stray return currents are necessarily much of a problem in DC systems as long as good modern insulation pads etc provided between running rails and bearers. That is the current state of the art for modern DC tramways to avoid stray current escaping into utility pipes and other surrounding metalwork, staying predominantly in the running rails which are often embedded in a continuous narrow trough in roadway sections backfilled with an insulating material. I have a theory that London's early decision to opt for a four rail power system was as much concerned with isolating traction current from the track circuits of signal systems, which in 3rd rail installations require expensive high current impedance bonds to separate traction and signalling current and which under certain fault conditions can result in high traction currents being injected into signalling equipment, often with fiery or explosive results, which would be very dangerous indeed underground. Metro systems were among the first adopters of continuous track circuiting and colour light signals.

For Basingstoke, if the freight locomotives and passenger trains being used that travel through Basingstoke towards Reading had a short distance battery pack built in and you had the start of OHLE about a half mile north of Basingstoke station, would that not resolve the issue?

The battery pack could have power regenerated to it both from electricity from the OHLE as teh train move along and from regenerative braking. That way you would not be needing the expensive kit as you have at Blackfriars/City TL.

 

[AM9]

I've sometimes wondered whether in mixed ac/dc areas, the dc sub-station nearest should be fed from the 25kV supply, so that everything comes off the one transformer (and phase). It would mean a lower transformer tapping than for the 33kV but most units have -15%.

Any mileage?

WAO

I've sometimes wondered whether in mixed ac/dc areas, the dc sub-station nearest should be fed from the 25kV supply, so that everything comes off the one transformer (and phase). It would mean a lower transformer tapping than for the 33kV but most units have -15%.

Any mileage?

WAO

Not really. Creating the DC from an auto transformer connected across the 25kV would still produce a DC earth current, and mean half wave rectification - the resultant harmonics would give immense immunity issues in lineside equipment and the DC traction equipment would need significant modifications to ensure satisfactory operation of a DC bus.

 

[zwk500]

For Basingstoke, if the freight locomotives and passenger trains being used that travel through Basingstoke towards Reading had a short distance battery pack built in and you had the start of OHLE about a half mile north of Basingstoke station, would that not resolve the issue?

The battery pack could have power regenerated to it both from electricity from the OHLE as teh train move along and from regenerative braking. That way you would not be needing the expensive kit as you have at Blackfriars/City TL.

You would wire into the Bay at Basingstoke with potentially one or two through platforms for the Reading-Basing shuttle.
Basingstoke to Southampton is about 65km, which is right on the limit of battery operation (but not impossible), so you'd need some serious charging infrastructure at Southampton and careful diagrams to avoid the loco fizzing out while shunting.
However batteries are probably the solution to Southampton freight either way.

 

[Bald Rick]

For Basingstoke, if the freight locomotives and passenger trains being used that travel through Basingstoke towards Reading had a short distance battery pack built in and you had the start of OHLE about a half mile north of Basingstoke station, would that not resolve the issue?

The battery pack could have power regenerated to it both from electricity from the OHLE as teh train move along and from regenerative braking. That way you would not be needing the expensive kit as you have at Blackfriars/City TL.

That would work, but you’d need a gap a bit longer than half a mile (I forget what the standard says about how far from existing electrification you need to immunise signalling for, but I think it might be 2km).

Also, if you’re having a battery system installed, you may as well make it a big one.

 

[Ken H]

That would work, but you’d need a gap a bit longer than half a mile (I forget what the standard says about how far from existing electrification you need to immunise signalling for, but 8 thin’ it might be 2km).

Also, if you’re having a battery system installed, you may as well make it a big one.

Any implications with batteries for signalling?

 

[MarkyT]

That would work, but you’d need a gap a bit longer than half a mile (I forget what the standard says about how far from existing electrification you need to immunise signalling for, but 8 thin’ it might be 2km).

2km from end of OHLE going into non-electrified terrritory for signalling immunisation rings a bell with me too, and I think I recall needing double IBJs on both rails at the limits to prevent any traction return current escaping into non-immunised equipment, even if a single IBJ fails. Modern equipment isn't usually a problem as most of it is intrinsically dual immune and mostly uses axle counters for train detection today which don't have any electrical commonality with the traction return current in the rails like older track circuits. Basingstoke, a fairly recent job in the early 2000s, uses track circuits throughout however. I questioned that at the time as Portsmouth was being designed contemporaneously using axle counters throughout, but the decision had been made. Can't be sure but the TCs used might be dual immune theoretically, although the system as a whole would probably require some hefty rework for the presence of wires anywhere nearby, as application design can differ according to site characteristics. There are techniques for coping with the earthing dichotomy in mixing AC and DC. A useful method is to provide massive reinforcement of return conductors; big cables paralleled with the rails essentially, frequently bonded, creating a current drain making the 'official' return path as low resistance as possible so any alternative stray path back to the substation is far less attractive to DC current flow. This method is expensive though so practically needs to be limited to short changeover sections. Axle counters can be effective in improving return current path conductance as they require no insulation between opposite rails which can be frequently bonded together to form a single larger current return conductor across individual rails of a single track and across multiple parallel tracks, with no need for impedance bonds.

 

[Meerkat]

So are you saying that even wiring into the Basingstoke bays would require a lot of immunisation to the whole station?
The length of Basingstoke-Worting is a bit of a problem if you don't want to convert all the way to Southampton but want to electrify to Salisbury (and maybe round to the port the long way for overhead).
Can you switch AC/DC at linespeed if you left Basingstoke-Worting as DC?

 

[zwk500]

So are you saying that even wiring into the Basingstoke bays would require a lot of immunisation to the whole station?

Basingstoke is certainly immunised against DC up to about the first overbridge on the Reading branch

The length of Basingstoke-Worting is a bit of a problem if you don't want to convert all the way to Southampton but want to electrify to Salisbury (and maybe round to the port the long way for overhead).

A 5km battery wouldn't be the end of the world, and you could have a feeder station at Salisbury so you'd just pan down into Basingstoke and use the battery (which you'd need anyway for shunting the docks) until you get back under OLE. Passenger Units would be coming from the Waterloo 3rd rail anyway so there's no AC > DC > AC quick changes.

Can you switch AC/DC at linespeed if you left Basingstoke-Worting as DC?

No reason why not, if you tension the OLE correctly.

 

[WAO]

Not really. Creating the DC from an auto transformer connected across the 25kV would still produce a DC earth current, and mean half wave rectification - the resultant harmonics would give immense immunity issues in lineside equipment and the DC traction equipment would need significant modifications to ensure satisfactory operation of a DC bus.

Thanks.

I was thinking classic, rather than AT.

MarkyT's post highlights the DC leakage problem which is there, AC or no AC.

Perhaps his description of a return cable from c2km from the boundary might lead back to a local centre return 4th rail, if that were conductive enough, even without direct connection to the rolling stock.

WAO

 

[MarkyT]

Modern track circuit equipment used on DC infrastructure is often also at least partially immune to AC traction current because it already has to deal with ripple in the DC supply, arising because the DC is rectified from AC in the first place. Ripple can be 50Hz and various harmonics related to it depending on the precise form of rectification. Simple track circuits designed for dual immunity usually operate at 'odd' frequencies, for example 83 (ish) Hz which are chosen specifically to be safely clear of mains harmonics. I think Basingstoke mostly used HVI (high voltage impulse) track circuits which use differing pulses of current that have to be received at the relay end in the correct sequence and timing constraints to register clear. They are dual immune intrinsically, but subject to varying application rules depending on type of electrification (e.g. max length, details of bonding cables, type, number and locations of impedance bonds, etc). If I was specifying it I'd require all train detection in the changeover area to be swapped for axle counters and all running rails bonded up together to provide a massively parallel redundant return conductor network, with reinforcement conductors added too, as expedient.

 

[Wolfie]

If there were a sincere, valid objection to 3rd rail then there would be a program, even if slow, of replacement, rather than a bottom-covering objection to small extensions and a mis-application of electrical safety rules (which previously exempted railways).

However I agree with a PW charge-hand friend (and his wife) that DC is less than desirable and suggest that, even if there is completion of the DC network (North Downs, Uckfield, etc), a start is made on replacement.

The two top projects for me would be AC from Pirbright Junction to Weymouth, as this is a fast route that also carries freight and is demanding of a DC system. My second would be Merseyrail's tiny 76 route miles. This tends to isolate suburban Liverpool/Merseyside at present, a state of affairs inimical to a trading region. The tunnel problem would be solved either by the new battery stock or by use of the ROCS system, which might just fit in the 4.7m dia tunnels.

Some chance!

WAO

Why Merseyrail when that system is broadly analogous to the London Underground where there are zero proposals to replace the four rail system?

 

[Dstock7080]

Wasnt there one tube line that had the rails in reverse polarity compared to the rest of the system? Think the Central London Railway. Something to do with earth leakage to the cast iron tunnel lining. The Central Line tunnels are smaller than the Yerkes tubes.
Apologies if I have misremembered.

The Central London Railway used a centre 3rd positive rail, until converted to LT standard in 1940.

 

[Bald Rick]

Any implications with batteries for signalling?

Only with frequencies produced by the motors / inverters on the train. Not for return current, as that stays in the train.

So are you saying that even wiring into the Basingstoke bays would require a lot of immunisation to the whole station?

Certainly lots of bonding changes. Otherwise the whole SWML DC return current would be trying to get back through the AC return path.

Can you switch AC/DC at linespeed if you left Basingstoke-Worting as DC?

Yes.

 

[WAO]

Why Merseyrail when that system is broadly analogous to the London Underground where there are zero proposals to replace the four rail system?

I agree. LUL seems to have ORR looking the other way. It has a lot of surface track, with twice the contact area above touch voltage than the third rail system.

Merseyrail DC requires (at present) buffer stops at its limits - fine for internal travel but a hindrance to expansion or integrated travel outside the region, which the local economy needs.

WAO

 

[alf]

Isn’t the third rail 25kv problem being over egged?

6 heavily used TfL dc Metropolitan/jubilee line tracks cross 4 busy 25kv WCML tracks in the Wembley area, & the 25kv lines themselves parallel third/fourth dc/London overground Bakerloo lines for 12 miles to Euston. This has been the case for over 50 years without fuss.

The 25kv Southend Fenchurch st line parallels the Upminster 630v dc line for 16 miles plus. No fuss.

The 4 track 25kv GWR main line at Ealing Broadway parallels the dc Central & District Line for a mile & is then crossed at right angles by the dc Piccadilly lines & district No fuss.

And 2 miles towards Paddington the Hammersmith & City Line parallels & dives under 6 25kv tracks on their final approach to Paddington. No fuss.

What is all this about?. There is no problem! It is routine.

And 132kv national grid power lines cross & parallel 25v & Third Rail dc lines throughout the land without fuss or comment.


Lus.