S Stock Acceleration Performance Question

[OneTrackMinded]

Why in manual does the S Stock have reduced acceleration?

Since the implementation of CBTC in large areas of the SubSurface network we've been able to see the full potential of the trains, operating at their maximum specified 1.3m/s^2.
Yet the question still remains why the cap has to exist on the traditional signalling.

During the early days of the trains, it was logical to have it capped in order to not catch up with the older stock that were still in service at the time, as well as lower voltage in some areas, but now that both factors have been removed from the equation, it makes little sense.

Such a question might not have to be asked if it was not for the de-scoping of the 4LM project, meaning for example branches on the District line will not receive the upgrade, which TfL has said means there can't be any journey time improvements in these areas.

Do LU just not trust their drivers enough to let them use the max acceleration? Or is something else at play here? I've been wondering this for years and it would be great to get some sort of answer even if that ends up being some technicality or similar.

 

[notverydeep]

They are limited to an acceleration profile that matches C / D stock performance when on conventional signalling. This is so that signal overlaps cannot be compromised by an over design-speed passing of a signal at danger. This is the only rationale for performance limiting new trains. Victoria line 2009 stock used an overlaid new signalling system from the start and were able to use the full performance, even while operating with 1967 stock working other services.

 

[OneTrackMinded]

They are limited to an acceleration profile that matches C / D stock performance when on conventional signalling. This is so that signal overlaps cannot be compromised by an over design-speed passing of a signal at danger. This is the only rationale for performance limiting new trains. Victoria line 2009 stock used an overlaid new signalling system from the start and were able to use the full performance, even while operating with 1967 stock working other services.

Thanks for the detailed response - although I wonder why speed limits aren't adjusted for such parameters instead or in conjunction

 

[edwin_m]

Thanks for the detailed response - although I wonder why speed limits aren't adjusted for such parameters instead or in conjunction

Under traditional LU signalling the speed limit is only enforced where a there is a timed tripcock. A high-performance train can accelerate to a much higher speed than permitted, either where there is no tripcock or even after passing it at below the permitted speed. Hence it can pass the signal much faster than it was assumed when designing the signalling, and be unable to stop within the overlap when tripped. High rates of acceleration on modern stock were one of the causes of the recent overspeeds at Peterborough.

Under TBTC the train is continuously supervised, so won't be allowed to exceed the permitted speed and will also be supervised down to a stop short of any train in front or junction where no route is set.

 

[Dstock7080]

Under traditional LU signalling the speed limit is only enforced where a there is a timed tripcock.

Under legacy signalling S Stock are limited to a maximum speed too, (MET) 60mph; (District) 47mph in weak-field areas and 42mph in non-WF areas.

 

[edwin_m]

Under legacy signalling S Stock are limited to a maximum speed too, (MET) 60mph; (District) 47mph in weak-field areas and 42mph in non-WF areas.

Is that limited by something on the train or simply an operating limit to be obeyed by drivers? If the latter, then there's nothing to stop a train being accelerated to a higher speed.

 

[Dstock7080]

Is that limited by something on the train or simply an operating limit to be obeyed by drivers? If the latter, then there's nothing to stop a train being accelerated to a higher speed.

Electronically limited to those speeds, it was not possible to get any higher even with the TBC at maximum.

 

[notverydeep]

There was some modification of the conventional signalling on the Metropolitan line, both to allow for the speed to return to 60 mph (it had long been reduced to 50 mph for A Stock) and to allow the faster C Stock acceleration profile, which while not as quick as the CBTC profile was an improvement on the much slower A Stock.