Strange Signal

[phil281]

The other day I was travelling from Twickenham to Bristol TM. After we'd just left Reading I was sitting facing backwards on the side facing the up line. We'd been going probably 5 or 10 minutes and I saw a very strange signal. It was like two signals strapped together, the 2nd one seemed to be a bit below and to the left of the main one. The main one was showing green and the other one a single yellow. We were going very fast so I may of been seeing things but can anyone shed any light on this? I didn't know such things existed. Is it some sort of distant because of the high speed on that line?

 

[The Planner]

Splitting distant for a diverging route. Im sure there is one at Tilehurst in the up direction.

 

[455driver]

Are you sure the signals were for the same line?

It sounds like a standard bracket signal with the top signal being for the up fast line and the lower signal being for the up slow.

 

[phil281]

Splitting distant for a diverging route. Im sure there is one at Tilehurst in the up direction.

Thanks! That must have been what I saw. How does it work?
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Are you sure the signals were for the same line?

It sounds like a standard bracket signal with the top signal being for the up fast line and the lower signal being for the up slow.

Almost 100%, the second signal looked different from what looked like the main one.

 

[Tomnick]

Clive Feather has an excellent explanation of how splitting distants (and other arrangements at junctions) work here.

 

[MarkyT]

Are you sure the signals were for the same line?

It sounds like a standard bracket signal with the top signal being for the up fast line and the lower signal being for the up slow.

Not applicable on western approaches to Reading as the 4 tracks are arranged (left to right looking west) DM-UM-DR-UR. Note main & relief, not fast & slow.
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Clive Feather has an excellent explanation of how splitting distants (and other arrangements at junctions) work here.

The splitting distants were introduced to speed up the approach to the improved ladder crossovers between Tilehurst station and Scours Lane as well as the turnout to platform 8 (old 5). As Clive Feather explains, it's not allowed to have overlapping 4 aspect flashing yellow sequences in such cases, so a splitting distant was the only option.

 

[phil281]

Clive Feather has an excellent explanation of how splitting distants (and other arrangements at junctions) work here.

Thanks Tomnick, is it only on the GW main line these are in operation? How long have they been around for?

 

[Tomnick]

They're quite an old concept, with plenty of semaphore examples in years gone by. The last one, around Larbert in Scotland, was recently abolished, I believe. Apart from this installation, and possibly one or two others further east on the GWML, I'm not aware of any other 'modern' splitting distants though.

 

[swt_passenger]

Do the 'arrow type' preliminary route indicators supersede the requirement for splitting distants on new installations?

 

[Tomnick]

My understanding of preliminary route indicators (and a quick look at the design standard, here, supports this - 5.14 on p120) is that they're only intended to provide routing information to reduce operational inconvenience from wrong-routing rather than any form of approach control to ensure that train speeds are reduced appropriately for diverging routes. This might mean a PRI being provided in rear of a splitting ("inner") distant in a four-aspect area, where there's sufficient distance from the latter to enable speed to be reduced for the diverging route but not enough to allow the train to come to a stand at the junction signal if wrong-routed.

 

[swt_passenger]

Thanks for the link - I'll have a good read of that.

 

[ainsworth74]

Really interesting link, thanks for that!