From earlier in this thread:
It will mark the final time steam trains travel into central London on the Underground due to signalling modernisation for the Circle, District, Hammersmith & City and Metropolitan lines from 2021.
An update from
London Connections on that signalling modernisation. A very long article which starts with an overview of the background
A much needed upgrade
New lines are always more exciting than upgrades to existing lines. On the Underground, the primary objective of any upgrade is nearly always to provide a capacity increase. This is certainly one of the main objectives of the SSR upgrade which is formally referred to as Four Lines Modernisation (4LM) and encompasses the Metropolitan, District, Hammersmith & City and Circle lines (also known as the Sub-surface Railway or SSR).
A major part of the upgrade of the Sub-surface Railway is the replacement of rolling stock. This phase has already been completed. Much needed as the new rolling stock was, it didn’t do that much to increase capacity. The significantly longer trains (an extra carriage length) on the Circle and Hammersmith & City lines certainly help. The extra standing space inside the carriages also helps – albeit at the price of a reduced number of seats per carriage.
The signalling factor
Another objective of the upgrade is to replace the old, unreliable signalling. But it makes no sense these days to replace signalling on a like-for-like basis. Whilst you are replacing the signalling you might as well improve it, where there are quantifiable benefits in doing so.
To get the full benefit of the substantial increase in capacity on the SSR you really need automatic train operation. Normally there is the additional cost of the extra trains required but as the fleet size of the SSR stock (S Stock) was calculated assuming ATO▸ would be introduced, there is no need to buy additional rolling stock in order to increase capacity. The stock is already there sitting in the depots.
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