valleyquay
New Member
Why don't they use the tripcock system on overground sections of Merseyrail?
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Why don't Merseyrail use the tripcock system, as used by London Underground?
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Bletchleyite
Veteran Member
Because they use TPWS?
bramling
Veteran Member
Firstly, it’s something else to maintain, and a nuisance one at that being a mechanical device.
But the bigger issue is that tripcocks don’t in themselves prevent a collision. They can only do that if provided in collaboration with a full-speed signal overlap, which has never been mainline practice. Indeed even on LU difficulties have arisen over time as train speeds have tended to get faster, which has made it more awkward to design signalling which can provide both a full-speed overlap and maintain the desired line capacity.
In short, tripcocks work okay until you get to around 45 mph, beyond that they start to become a headache as the required overlap becomes so long it destroys the line capacity. Especially as overlaps have tended to get longer over time as standards have tightened.
I'm guessing the use of tripcocks in the tunnelled section is a legacy of the original construction of the tunnels, being so soon after the Moorgate accident (and no other alternative system being available at the time). Whereas the "open air" sections of Merseyrail were all existing railway lines (mechanically signalled in a lot of places)
bramling
Veteran Member
Yes it does seem to be the case that something had to be done for Drayton Park to Moorgate, and then Merseyrail essentially was deemed to be comparable.
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