Farringdon 4th rail

[hotelmode]

Anyone got any idea why a new 4th rail appears (in the normal place a la Underground) to have been installed on the FCC tracks at Farringdon? It looks like is fixed in place but i only saw it briefly before my train arrived.

 

[Edvid]

That 4th rail has been there for a while - it handles the return current I believe (a relatively complex setup is required for dual-electrified* lines).

The running rails, sleepers and I think the 3rd rail were recently renewed though.

[* 25 kV AC OHLE + 750 V DC 3rd rail]

 

[hotelmode]

That 4th rail has been there for a while - it handles the return current I believe (a relatively complex setup is required for dual-electrified* lines).

So just like the Underground.

Its definately a new one, and doesnt appear to have any insulators as per the old one (which just looked like an old rusty rail, I assumed it was a throwback). It also doesnt continue towards City Thameslink despite the overhead wires continuing to the tunnel portal, its only in the platforms.

Do any mainline trains actually have any means of returning current through a centre rail?

 

[Edvid]

I'm not aware of any mainline trains being equipped with 'shoes' running through the middle. Not that it matters seeing as the 4th and running rails are connected (certainly in Farringdon anyway).

The OHLE extension to City Thameslink isn't due to go 'live' until December 6, however the Moorgate branch won't be closed for another week afterward so the 4th rail may remain as it is.

 

[westcoaster]

i have never been told what it is for, it also does not carry any current there used to be an orange box at the end of the rail but that was removed when the track was renewed. so ill see what i can find out.

 

[asylumxl]

Hmm. Strange.

 

[O L Leigh]

That 4th rail has been there for a while - it handles the return current I believe (a relatively complex setup is required for dual-electrified* lines).

The running rails, sleepers and I think the 3rd rail were recently renewed though.

[* 25 kV AC OHLE + 750 V DC 3rd rail]

I'm sure Old Timer will provide chapter and verse on this, but my understanding is that it is neither complex nor specifically required for lines electrified with both OLE and CRE.

If it's part of the return current path, it's simply there as an additional conductor and, as such, it will be bonded to the running rails. In every other respect the return path will work in precisely the same way as elsewhere.

O L Leigh