"D7666" wrote

Todays Thamelink operations - and all those of the greater operation
post 2015 - seem to work OK without any resort to self propulsion.
There are no significant engineering blocks where diesel working would
be useful - lines tend to be closed outright rather than OLE
isolations, and the SR zone tend not to indulge in traction only
isolations.

It is the central sections BF-City-KX that would be most vulnerable to
a train failure. But the service will be so intense through there that
rather than **** around with alternative power (that will be so
infrequently used it will itself be a liability) you simply bring up
the train behind and push out.

And as others have quite rightly pointed out, the central section has
some fierce grades - that from City to BF is the steepest on the
network for practical purposes (although ?? might change with the
upgrade ??). Bearing in mind that an EMU with more than 50% motors cut
out will struggle up there, I don't see how piddly 200 hp power packs
will help any.

Since the new TL is supposed to be about longer trains, the chances of
a train with more than 50% out reduces - because to get to that state
with modules of 4car EMUs you need multiple motor failures across the
train affecting more than one unit. Thats pretty rare. Even with a
complete disablement of a whole train, you push out with a fully
functioning train, so unless a farce arises where it just happens that
it is a 4car following a failed 12car, probability suggests its a fair
risk to not bother with alternatives.

I have to wonder if the person who wrote this into the spec just
happens to be a TL commuter and got caught one day in one of those
rare events of an AC/DC changeover failure at Farringdon and has gone
for sledge hammers to crack nuts approach. Those failures are very
rare, I've never been involved in one in regular travel on the route
ever since it opened.

Whilst I agree with much of this, and am not convinced that auxiliary
traction power is justified, there is also the scenario where failure of the
power supply traps trains between stations. While stations are close
together between Farringdon and Blackfriars, so it is difficult to conceive
circumstances where more than one train could be trapped on each road
between each pair of stations, between Farringdon and Kentish Town stations
are more widely spaced. Kings Cross Thameslink has been retained as an
emergency evacuation location, but I can imagine the difficulties if say
three peak trains were trapped between Kentish Town and St Pancras, and 3000
or more passengers had to be evacuated on foot. It would be useful if
feasible to be able to move a train to a platform, or out of a platform to
clear it for a following train.

Peter