Future of Third Rail

[zwk500]

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

Vertically and horizontally adjacent tracks are easy to separate and are not a serious problem, it's when the same track has both systems that it's a problem.
And yes, it is a problem.

 

[HSTEd]

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.

When was the last time a live fourth rail installation was extended on the surface though? Excluding sections shared with National Rail with earthed centre rails.

It must have been a significant time ago.

Although myself I'd like to take advantage of the de-peakification (is that a word?) of the railway to extend the tube further, I don't think anyone in power shares that desire. So it may be a long time before the London Underground gets another surface extension.

 

[stuu]

When was the last time a live fourth rail installation was extended on the surface though? Excluding sections shared with National Rail with earthed centre rails.

It must have been a significant time ago.

Although myself I'd like to take advantage of the de-peakification (is that a word?) of the railway to extend the tube further, I don't think anyone in power shares that desire. So it may be a long time before the London Underground gets another surface extension.

Jubilee Line to Stratford. It wasn't raised as an issue when the Met Line was planned to be extended to Watford Junction either, oddly.

 

[AM9]

Jubilee Line to Stratford. It wasn't raised as an issue when the Met Line was planned to be extended to Watford Junction either, oddly.

The Jubilee tracks from Canning Town to Stratford are pretty well protected from intrusion, even via the DLR formation, so effectively their envelope of safety hazards is nothing like a virtually open 3rd rail line (especially where they have unmanned stations and at-grade crossings).

 

[zwk500]

It wasn't raised as an issue when the Met Line was planned to be extended to Watford Junction either, oddly.

Wasn't that extension largely on a viaduct?

 

[Bald Rick]

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.

Two points, both rather significant.

1) the main issue is where AC and DC trains run on the same tracks, ie where the AC and DC return currents have to use the same rails, AND there is high current draw on the DC side, ie frequent DC trains drawing lots of current. Hence why it is a problem at Farringdon / City TL / Blackfriars where there are 40 x 8/12 car trains drawing lots of DC an hour, but it’s not so much of an issue between Euston and Camden where there are 8 x 5 car trains an hour.

2) The LU system has 4th rail, which keeps return currents out of the running rails altogether, and it is therefore not a problem at all.

 

[HSTEd]

The Jubilee tracks from Canning Town to Stratford are pretty well protected from intrusion, even via the DLR formation, so effectively their envelope of safety hazards is nothing like a virtually open 3rd rail line (especially where they have unmanned stations and at-grade crossings).

Sometimes I despair about the non-prescriptive safety regulation system employed in the UK.

I know it has many advantages, but if the ORR/HMRI would just come out with a set of rules to be met in any new third rail installation, it would be a hell of lot easier for everyone to work out what might be possible and what would not be. It has a significant chilling effect when there is no certainty that a scheme won't go through expensive engineering studies just to get kyboshed by the regulator after a lot of money is already gone.

There are clearly engineering ways around the station platform/level crossing issues (for example, platform edge doors or gates on the level crossings that obstruct the track when the road is open)

 

[zwk500]

Sometimes I despair about the non-prescriptive safety regulation system employed in the UK.

I know it has many advantages, but if the ORR/HMRI would just come out with a set of rules to be met in any new third rail installation, it would be a hell of lot easier for everyone to work out what might be possible and what would not be. It has a significant chilling effect when there is no certainty that a scheme won't go through expensive engineering studies just to get kyboshed by the regulator after a lot of money is already gone.

There are clearly engineering ways around the station platform/level crossing issues (for example, platform edge doors or gates on the level crossings that obstruct the track when the road is open)

I believe the RSSB have published exactly this, but you need an RSSB login to access it. (Or a willing friend ...)

 

[HSTEd]

I believe the RSSB have published exactly this, but you need an RSSB login to access it. (Or a willing friend ...)

I think I've found it (T1214 'DECARB: 21st Century DC electrification infill').

Luckily, I have an RSSB login, and thus I will now consume this knowledge to gain its power and enable ever greater crayon outputs.

 

[zwk500]

I think I've found it (T1214 'DECARB: 21st Century DC electrification infill').

Luckily, I have an RSSB login, and thus I will now consume this knowledge to gain its power and enable ever greater crayon outputs.

That be the one! (It's an interesting, if dense, read)

 

[HSTEd]

That be the one! (It's an interesting, if dense, read)

One thing it notes is that the London Underground managed to make a case to the ORR that its practices were sufficiently risk-managed to render the four rail system as safe as "reasonably practicable".

Apparently key features of their case was that there are no level crossings on the system and little or no live working occurs trackside.
But the report seems reasonably optimistic that a case could be made in any case, at least from the tone of the summary documents.

 

[GRALISTAIR]

The LU system has 4th rail, which keeps return currents out of the running rails altogether, and it is therefore not a problem at all.

Slightly OT but that suggests to me that sparking up the Chilterns out of Marylebone will not be as big a challenge as some infer?

 

[LBMPSB]

It is a shame the Marsh Link isn't electrified now. Hastings would have had a direct service to London Charing Cross, instead of the inconvenience of Engineering Works affecting journeys this weekend.

 

[Bald Rick]

Slightly OT but that suggests to me that sparking up the Chilterns out of Marylebone will not be as big a challenge as some infer?

Yes, for AC electrification, on the issue of return currents, it’s not a big issue. Plenty of other issues though!

 

[LBMPSB]

Yes, for AC electrification, on the issue of return currents, it’s not a big issue. Plenty of other issues though!

There are several issues with Ac Electrification, such as immunising the track circuits and signalling systems from stray electical currents caused by OH power supply, similar issues exist with DC.

 

[stuu]

Wasn't that extension largely on a viaduct?

No, the major new bit was(~500m) but the rest of the branch was at ground level or in a cutting

 

[Mikey C]

One thing it notes is that the London Underground managed to make a case to the ORR that its practices were sufficiently risk-managed to render the four rail system as safe as "reasonably practicable".

Apparently key features of their case was that there are no level crossings on the system and little or no live working occurs trackside.
But the report seems reasonably optimistic that a case could be made in any case, at least from the tone of the summary documents.

I would suggest that nothing on the 3rd rail network is as dangerous for the public as some of the overcrowded Underground platforms, especially the narrow single platform stations Clapham North and Clapham Common

 

[Bald Rick]

There are several issues with Ac Electrification, such as immunising the track circuits and signalling systems from stray electical currents caused by OH power supply, similar issues exist with DC.

I was specifically referring to the issue of installing 25kV AC over LU tracks electrified on the 4th rail system, and that there is then no need to deal with bonding in the running rails for DC return currents (as there isn’t any).

 

[zwk500]

I was specifically referring to the issue of installing 25kV AC over LU tracks electrified on the 4th rail system, and that there is then no need to deal with bonding in the running rails for DC return currents (as there isn’t any).

Is there anything specific to the return Currents issue with 3rd and 4th rail trains on the shared track, or is it essentially the same as ordinary 3rd rail.

 

[AM9]

I would suggest that nothing on the 3rd rail network is as dangerous for the public as some of the overcrowded Underground platforms, especially the narrow single platform stations Clapham North and Clapham Common

I would suggest that tube train noisily entering a tube station at c.30mph, slowing to a standstill within 120m is nowhere as hazardous as a diesel or electric multiple unit travelling at 95mph through St Albans platform 3 when crowded. If there was any attempt to get approval to configure a station like that from scratch now, it would be thrown out by the RSSB before it hit their desks!

 

[WAO]

Yes. Kinetic energy, not electrical energy is the great danger on the railway.

4mph, preceded by a red flag, or we shall all be murdered in our beds!

WAO

 

[D365]

Yes. Kinetic energy, not electrical energy is the great danger on the railway.

My PTS (track safety) course consisted of track and electrical safety in equal measure, with a particular emphasis on third rail.

 

[Wolfie]

I would suggest that nothing on the 3rd rail network is as dangerous for the public as some of the overcrowded Underground platforms, especially the narrow single platform stations Clapham North and Clapham Common

I remember using the pre-rebuild Angel. That was similar but much busier and frankly was bloody scary.

 

[WAO]

Sandhills (Merseyside) when 4 road, with island platform, also!

WAO

 

[A0]

It is a shame the Marsh Link isn't electrified now. Hastings would have had a direct service to London Charing Cross, instead of the inconvenience of Engineering Works affecting journeys this weekend.

I don't think you can make any such assertion.

Marshlink is capacity constrained.

If you're just looking at crayoning lines covered on your rail atlas, there's nothing to stop you travelling that way even now - just means changing at Ashford.

 

[Bald Rick]

Is there anything specific to the return Currents issue with 3rd and 4th rail trains on the shared track, or is it essentially the same as ordinary 3rd rail.

the 4th rail is bonded to the running rail, to help encourage the return current into the 4th rail. Obviously there needs to be special arrangements for the track circuits.

 

[Pigeon]

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.

Yes, the Bakerloo was like that when first opened. They put the positive in the middle to reduce leakage problems, it is said. Elsewhere this was achieved by greater insulation. It does rather suggest that making negative in the middle the standard was basically doing it the wrong way round in the first place, and raises the question of why they (a) did do that and then (b) stuck with it and converted the Bakerloo to it, instead of adopting the Bakerloo polarity as standard. I think this is one of those things where the narrative has been obscured by being passed on through multiple stages of unscientific recounters, who don't understand that by eliding the technical considerations behind that bald summary they only serve to create puzzles for future readers.

Other early variations were the Waterloo & City, which used "standard" third rail until it became part of "the Underground"; the Central London, which used its own form of third rail; and the C&SLR, which used third rail with +500V in one tube and -500V in the other. Maybe they were trying to balance things so that differential leakage currents from one side to the other would be mostly confined to the volume immediately around and between the tubes, and common-mode ground currents would ideally cancel.

Fourth rail didn't start happening until Yerkes decided on it for his tube lines and the District, which sooner or later led to everyone else making themselves compatible.

 

[Ken H]

Remember the District was originally the Metropolitan District and was a child of the Metroplitan Railway. But the best laid plans and all that and the district ended up in the Yerkes Underground group.
The district electrified at the Yerkes standard but the Met wanted a different system for their lines, including the northern bit of the circle. They wanted some sort of overhead system but arbitration meant they had to install the Yerkes standard system. But not standard enough to stop the shoes falling off the narrower juice rails the Met installed!

 

[MarkyT]

Other early variations were the Waterloo & City, which used "standard" third rail until it became part of "the Underground"

4th rail was added in the early 1990s for when the class 482s were delivered, still under NSE, but not long before privatisation in 1994 when the line was handed over to LU. The 482s were short variants of the Central Line 1992 stock being constructed around the same time. Previous rolling stock had been able to travel to Eastleigh works for overhaul under its own power on the LSWR mainline using the 3rd rail.

 

[LBMPSB]

Is there anything specific to the return Currents issue with 3rd and 4th rail trains on the shared track, or is it essentially the same as ordinary 3rd rail.

With 4th rail, traction current is returned via the fourth rail. With 3rd rail, the traction current is returned through the running rails which is also shared with track circuit currents which can require special bonding, depending on type of track circuit.