Snaresbrook

[JG Morgan]

I was waiting at Snaresbrook last week for rather longer than expected and was surprised by the operation of the signalling system.
The only visible signals are a three-aspect "starter" at each end of the platforms. These show red when a train has just passed (as we would expect, of course), but then change to a white light in the middle aspect (where I would expect to see yellow). Then, after an extended period (a few minutes) that eventually changes to green.
I think (although I am not certain) that the signal at the south end of the station (westbound line, towards central London) "reverted" at one time from green to white. Is that normal or possible?

There did not appear to be any trip-cocks (or train stops) associated with either signal.

Can the Underground experts enlighten me, please?

[S&TEngineer]

You can find your answers here under the section half way down the page titled "SIGNALS": http://www.davros.org/rail/signalling/a ... ntral.html

[Mike Stone]

Norton is blocking that link.

[Ashley Hill]

Works ok on iPhone. Fascinating information.

[John Hinson]

Norton is blocking that link.

Curious. Not for me it isn't.

Still, if anybody knows the answer to the specific question it would be useful to see itsummarised here rather than have to wade through an entire web site to find it.

John

[Andrew Waugh]

Summarising as per John's request.

The line is equipped with automatic train operation. ATO operation is controlled by frequencies transmitted through the rail and detected by the train. The specific frequency received indicates the current maximum speed and a target speed.

A red aspect indicates stop (i.e. the frequency received will have a target speed of 0).

A white aspect indicates that a proceed code (i.e. a target speed greater than zero) is being transmitted. Trains operating automatically can pass, as can trains being manually driven but supervised by the ATO. Trains not under control of the ATO cannot pass.

A green aspect indicates that the line is clear *up to the next signal*. A train running under any mode (ATO, supervised, or totally manual) can pass.

I doubt that the aspects can regress from green to white.

andrew

[S&TEngineer]

Apologies about the link. It worked OK for me. The system described is the same as that developed for the London Underground VICTORIA line way back in the mid-1960s (and first commissioned in 1968) so its not a new system but the technology used to deliver it is completely different of course.

I have amended my original post to direct to the relevant part of the complete article (slap on wrist!).

[KEVIN SMITH]

As do not live far from Epping and I sometimes use the line I can also tell you that there are a great many block marker boards (Both controlled and Automatic ) along the line between signals. If there is a train somewhere between these signals and a train is operating in ATO then a white aspect tells the driver that the train will be brought to a stand at a block marker board . Not all signals can show a white aspect if there is complex point work ahead ( or in certain parts of the tube section) but most do . You will need to get yours hands on some old yellow perils ! for the line to find out how and where it is all laid out as there are no hard rules here it depends on location.

[scarpa]

The use of signals along the auto sections is confined to providing them at platforms and on the approach to station platforms. occasionally on long sections a colour light signal may be provided mid section in addition to the Block Marker posts .On Sundays Train drivers are allowed if they wish, to drive the train manually .This rarely happens. Other times if a failure occurs of the A.T.O. the speed of the train is limited to 18 m.p.h. The Control centre is at White City for the Central line .

[JG Morgan]

Thank you all.
I have been away on holiday, so today is my first time in the forum since posting the question.