Why does the S stock have such poor acceleration

[pethadine82]

I have noticed that the S stock despite increase in line speed has really poor acceleration. The 96 stock on the jubilee line and even central line stock has better acceleration.
Coupled with LU mantra of all stations at all times makes for very long journeys.

Why did they not think of increasing the line speed on the open sections of the met line to 75mph which would result in quicker journey times for the chiltern line trains.

Would it have been easier to convert the subsurface lines to class 378 which may have worked out cheaper overall?

 

[rebmcr]

The S Stock is awaiting older stock to be withdrawn. They currently run at a compromise of 630v (+410v / -220v across the third & fourth rails), when the upgrade is done in a few years they will run at 750v (I believe +750v / 0v).

 

[Domh245]

The S stock still have better acceleration than the stock that went before them, and I think that they are still operating at slightly reduced power, because there are still "legacy" trains (read D stock) operating, and so until they are gone, they are being made to operate at a reduced performance to prevent the service bunching up in the central areas. Not only this, but the trains are still being driven by a driver, whereas the central and jubilee lines are ATO - which is another factor behind the slower acceleration. Raising the linespeed to 75 would have had no benefit for the met line, as their trains can only do 62mph tops, and to increase the linespeed by 15mph would have required signalling to be replaced, which would have led to a lesser capacity overall.

As for converting to 378 - that would have been very difficult, as the 378 is a 3rd rail EMU, whilst the S is a 4 rail EMU. The 378 would also have meant that there would be less space inside the train, and it would be slower operation - so to convert to 378 operation would have been very expensive as you would have had to completely redevelop the train, or the infrastructure, or both!

 

[Met Driver]

The S stock still have better acceleration than the stock that went before them, and I think that they are still operating at slightly reduced power, because there are still "legacy" trains (read D stock) operating, and so until they are gone, they are being made to operate at a reduced performance to prevent the service bunching up in the central areas. Not only this, but the trains are still being driven by a driver, whereas the central and jubilee lines are ATO - which is another factor behind the slower acceleration.

You're absolutely right in the first sentence. I'm a bit confused by the second. Are you suggesting that drivers would be incapable of utilising the full acceleration potential, were it available? The shortcomings which prevent the trains being manually driven at full performance lie with the current signalling and not with the competence of the drivers.

When driven manually, the 1992 stock is capable of the same performance as when operating in ATO.

 

[waterboo]

Some of this information has been sourced from Wikipedia:

Class 378 Width 2.80 m
Standard stock width 2.92 m

Class 378 formation 5 cars maximum
Standard stock formation 7 or 8 cars.

Class 378 Doorways 4
S stock doorways 6

So can see that S stock is better designed for a very, very busy service.

 

[edwin_m]

As for converting to 378 - that would have been very difficult, as the 378 is a 3rd rail EMU, whilst the S is a 4 rail EMU.

I don't think this is a reason for it to be difficult - surely just a question of adding extra shoegear and re-routing the negative feed to the traction equipment?

However I agree there would be plenty of other reasons not to use the 378 on the sub-surface lines.

 

[Nym]

The new ATO and driving profiles in the encoders limit PM performance below that of ATO performance for 09TS and S Stock.

 

[pethadine82]

Someone made a comment about the voltages and that LU have some at - something and the other at plus something.

If the potential difference is 750, what difference does it make if they change the inner rail to 0 volts and the outer to 750 since it still makes 750v.