How / What / Why?

Ever wondered the reason for something on the Underground?

Perhaps a few of your questions will be answered here

1   When I am following a train, why does the SB starter at Highgate take so long to clear?

The section of starter A513 is approx. 3240 feet long, ending at the start of the 25 mph speed restriction

This means that the cab of the train in front has to have almost reached repeater RNN14/15a before the starter can clear. This is about the same distance as that between West Finchley and Woodside Park.

2   Why is there a wide tunnel mouth at the north end of Kennington  NB platform 3 and a wide tunnel mouth at the south end of Elephant & Castle NB platform?

In City & South London Railway days, all the C&SLRly platforms between Clapham Common and Euston were approx. 200 feet long and straight. They were later extend to the existing 7 car length.

At both Kennington and Elephant and Castle platforms, the platforms were extended southwards. The extension can be clearly seen by the difference in shape of the platform tunnel. The extended part of the tunnel is created in tunnel segments and is round and slightly larger. The original part of the tunnel was built in brick and had a slightly oval appearance.


Originally, the platform and track were on opposite sides. In order to allow for cross-platform interchange with the Charing Cross branch, the platform and track were swapped over. The original run in to the platform would have been straight, not on a curve like it is today. The original exit was on what is now the track wall side - the shape of the blocked off exit can still be seen at the north end of the platform.

The tunnel mouth had to be widened in order to allow the diverted track to rejoin the existing track.

Elephant & Castle

Again, the platform and track were originally on opposite sides. They were swapped over to allow interchange with the Bakerloo Line via the new entrance at the north end of the platform. Again the tunnel was widened to accommodate the change in direction.

The original exit was via an exit at the rear of the station tunnel, which is now where the track is.

3   What is the purpose of signal U100 in Stockwell NB platform? - it always seems to be yellow

U100 is a draw-up signal associated with starter U10. Inner home U11 initially protects the crossover.

If there is a train at U5 reversing N - S, this is occupying U11’s section. A following train would be held at U11 until the move is completed and the points returned to normal.

In these circumstances, to allow the following train into the platform, U11 becomes a speed signal and clears when the speed of an approaching train is reduced. U100 remains at danger and now protects the crossover. U100 will now act as a speed signal and clears when the train’s speed is around 15 mph, allowing the train as far as U10. U10 now protects the crossover.

4   Why is J11 (Kings Cross NB inner home) sometimes at danger, even though I haven’t been following a train?

J11 protects crossovers 6 and 7. Crossover 7 allows trains in and out of the Kings Cross loop, Crossover 6 lets trains out of the Euston loop.

If a train is to go in or out of Kings Cross loop, J11 will be held at danger until the move is completed and the points have returned to normal.

An alternative is to wait until the train has stopped in Kings Cross platform. Starter J10 will now be protecting the crossovers and the move can be made.

5   Colindale NB home signals A434a and A434a are automatic signals, yet their normal aspect is red.

Why is this?

When Colindale was opened, AC1 was the only home signal and it protected a train in the NB platform. It was in the position that outer home A434a is now. At a later date, AC1 became A434a and two more signals were added - A434a the intermediate home, and an inner home which was labelled AC1. Draw-up signal A4340 was also added.

Provided the relevant sections are clear, A4340, A434a, A434b and AC1 normally clear when the approaching train passes the blockjoint at A432. I do not know the exact reason why these signals have a normally red aspect, but I suspect it’s a legacy from the old days, If you know the definite answer, then please tell me!

6   What is the sign under the rail gap indicator north of B31a at Kennington for?

The RGI lets the driver know that traction current is discharged on the Charing Cross branch at Kennington and not to proceed. Loss of traction current on the Charing Cross branch has no effect on the Bank branch - that current section is from Stockwell to Elephant & Castle. The sign illuminates at the same time as the RGI. In effect, it tells the Bank branch drivers that the illuminated RGI only applies to trains going via Charing Cross and Bank branch trains should proceed as normal.


7   Where does the passageway lead to on your left as you leave Stockwell NB, just as you come to the crossover tunnel?

The passageway is now blocked off from the trackside. Stockwell was the terminus of the C&SLRly and had an island platform. Access to / from the lifts was via this passage. When the railway was extended to Clapham Common, a stairway was built from the south end of the platform to a new passageway (where the existing walkway is today). When the line was extended to Morden, new platforms were built and the old platform area now has the crossover in it. The station tiles can still be seen on the walls and ceiling of the crossover tunnel. The diagram shows three stages of track layouts at Stockwell.

8  Why does the disused City Road station have a short platform area?

All the C&SLRly platforms were around 200 feet long.In preparation for the extension of the line to Morden and up to

Camden Town, the existing tunnels were widened and the platforms were extended to 350 feet long. Meanwhile, it was decided that there was insufficient traffic for the station to remain open, and it was closed without the platforms being extended.

9  What is the switch on the tunnel wall near 16 points, at the junction between the NB Bank branch and NB Charing Cross branch for?

This is a circuit breaker (there are two back to back). They are normally open, but can be manually closed to couple the Kennington to Lambeth NB and the Stockwell to Elephant & Castle NB traction current sections together.

10.  Between Kings Cross and Euston NB (near 7a points) there is a signal phone at the crossover and further on on the NB main there is a 7-car mark. What are they for?

Before the NB diversion was built, trains for the Kings Cross loop used to reverse N-S on the original NB main (now the Euston loop). During the diversion works, the stopping mark was put in the new tunnel and trains would stop there to reverse.

The signal phone is where the shunt signal used to be.  

11  What are the REFUGE signs that are near the signals in both tunnels between Archway and East Finchley?

These signs indicate the nearest bolt hole or cross passage that can be used as a place of refuge if a train was passing. These would not normally be used as such during traffic hours.

They are at every signal on the northbound and most signals on the southbound.

12  What is the door in the wall at Clapham Common SB for?

Clapham Common was originally the terminus with a 200 feet long island platform with a scissors crossover at the north end.

There was also a siding leading off from the southbound main. When the platform was lengthened northwards, the station tunnel, being wider, cut into the siding. The remains of the siding was then blocked off, with a door provided for access.

a diagram will be added later

13  Why do the motors sometimes drop out, or the train jolt when leaving a station?

This usually occurs where there has been excessive arcing on the negative rail which, in effect, creates additional rail gaps.

Diagram. Photo. Diagram. Diagram. Diagram. Photo 2. Photo 1. Photo. info. here.