Bi-mode Lines

[Class15]

Because the DC return current uses the fourth rail, rather than the running rails in the case of third rail electrification

So hypothetically, electrifying Harrow-on-the-Hill to Amersham at AC for Chiltern in addition to fourth rail wouldn’t be that bad?

 

[rower40]

So hypothetically, electrifying Harrow-on-the-Hill to Amersham at AC for Chiltern in addition to fourth rail wouldn’t be that bad?

And it would supply its own electricity, by attaching generators to the now-spinning graves of the LUL signalling engineers…

 

[edwin_m]

Indeed. 4th rail + OLE is much easier to accommodate than 3rd rail +OLE.

Because the DC return current uses the fourth rail, rather than the running rails in the case of third rail electrification

I expect Old Dalby could easily be configured for third rail instead, given the high probability of stock needing to be tested on that system.

 

[Elecman]

The primary problem with mixed ac and 3rd rail dc systems is how traction return current is dealt with.
With ac systems the rails are connected to every metallic structure through bonding cables to keep the Earth return path resistance and thus voltages down as close to zero as possible.
With dc the rails are insulated from Earth as far as possible to keep the Earth resistance as high as possible and stray return current leaking to earth as low as possible. Dc current leaking to earth is liable to cause serious corrosion damage over time to any nearby metallic structures/pipes

 

[FOH]

Yep, 110mph (or 50mph wrong way), looks like the 3rd rail only stretches 50m into the 285m long platform - I can't imagine it has seen much use.

IIRC it used to be used a fair bit to get 313s off the DC and up to Bletchley where they were maintained.

 

[thenorthern]

Around Allerton TMD I think there may be some dual overhead and 3rd rail but this is of course, not used by passenger trains in service.

 

[Paul AC]

Mention has been made above of the four-rail electrification between Queens Park and Harrow & Wealdstone. However, as this section is used by both three-rail London Overground trains between London Euston and Watford Junction, and by four-rail London Overground Bakerloo Line trains, the centre conductor rail is bonded to the running rails. The four-rail electrified section actually extends into the turnback siding at Harrow & Wealdstone, and 'back' towards Euston from Queens Park as far as Kilburn High Road.

 

[a_c_skinner]

What are the arrangements on Thameslink? Hasn't northbound changeover moved from Farringdon to City Thameslink? Does that leave a small length of both on northbound?

Edit: Got my north and south mixed up see below.

 

[Bald Rick]

What are the arrangements on Thameslink? Hasn't northbound changeover moved from Farringdon to City Thameslink? Does that leave a small length of both on northbound?

There is OLE and third rail on both lines between (and including) Farringdon station and City Thameslink station.

Changeover is at the first opportunity, ie Farringdon southbound and City TL northbound.

There are some heavy duty power electrical equipment in various locations on that section (and out to Blackfriars) to manage the return current. Even then, parts of the track system, namely the insulated block joints between City TL and Blackfriars, wear / burn out at a rapid pace. Most recently last week (south end of platofrm 1 at Blackfriars).

 

[nw1]

Because the DC return current uses the fourth rail, rather than the running rails in the case of third rail electrification

Does this mean that you can get a shock from the running rails on third-rail lines where a third-rail exists?
Apologies for my ignorance on how this all works.

 

[Belperpete]

On the WCML proper, they are mostly Ac immune. The ammount of DC return around is small, not least becasue fo the 4th rail south of Harrow.

There may be a 4th rail, but aren't the running rails still used for the traction return for the overground stock? If so, how is the amount of DC traction return small?

 

[Beebman]

Around Allerton TMD I think there may be some dual overhead and 3rd rail but this is of course, not used by passenger trains in service.

When the Liverpool area was 25kV electrified circa 1960, the CLC line was wired from Hunts Cross West Jct to almost Hunts Cross Station and AIUI this allowed an 8-car EMU to or from Allerton Station to access the far (eastern) end of Allerton Depot via a reversal. My understanding is that by the 1980s this was hardly ever used and when the Merseyrail 3rd rail was extended to Hunts Cross this section of 25kV OLE was disconnected and isolated (but not removed), maybe to avoid current return issues. More recently this OLE was finally removed, however a new 25kV electrified headshunt siding was installed alongside the DC lines at the eastern end of the depot. Although there are no dual-electrified tracks here, presumably the proximity of the two systems to each other must still require some return current management?

 

[Bald Rick]

There may be a 4th rail, but aren't the running rails still used for the traction return for the overground stock? If so, how is the amount of DC traction return small?

The running rails are bonded to the 4th rail, and the traction return typically flows through the latter as it has lower resistance.

 

[norbitonflyer]

Depending on how you define dual electrics, Kilburn High Road to Harrow & Wealdstone is electrified with both third rail and four rail systems

The two sections of the District Line shared with NR trains (the Richmond and Wimbledon branches) are also arranged this way. The centre rail is there, and used for current return by Underground trains, but it is bonded to the running rails so it is at zero potential instead of, as is usual on the rest of the Underground, at about minus 220. (the positive riai is at +660V instead of +440V, so the train sees the same 660V differece in either case.

== Doublepost prevention - post automatically merged: 12 May 2026 ==

Yep, 110mph (or 50mph wrong way), looks like the 3rd rail only stretches 50m into the 285m long platform - I can't imagine it has seen much use.

It was installed when Croxley Green depot closed and 313s took over the Watford DC service from 501s. These were maintained at Bletchley depot, so there was a need for a transmferr ac/DC chageover point. (they also worked the Abbey Flyer)