PowerSwitch

Not in fact the first ever as has been widely claimed, but a most welcome forward step.
These trains are powered either by the existing live rail, or on lines not electrified, by a battery. These trains are particularly useful in areas such as Merseyside, or on the former Southern railway, that are electrified with a live rail. The health and safety industry have in effect prohibited any significant expansion of live rail infrastructure.
Therefore any expansion of services is reliant on either diesel power, or on expensive dual voltage trains, and electrifying the new bit at 25 kv overhead.

https://www.youtube.com/watch?v=O6Kz8pbjF50

GWR have introduced an experimental battery train on the greenford line . It uses fast charging through a short centre 3rd rail whilst in the station at greenford. Four minutes at 2MW is enough to charge the battery to run the train to the other end of the line and back. To provide 2MW without overloading the local grid, the power is provided by a second large battery in the station, and this in turn is charged much more slowly at a lower power rating from the grid, ready for when the train returns for its next change.

This is a cheap way to electrify short branch lines and could be extended with extra charging points in stations along a route. Obviously it is not going to work for long distance or high speed routes. It will be interesting to see what the working life of the batteries is on this train.

The new trains on Merseyside have had a very problematic introduction....

Liverpool train delays 'frustratingly poor', Mayor Steve Rotheram says
https://www.bbc.co.uk/news/uk-england-m ... e-67353857

Battery-powered train disruption refunds cost revealed:
https://www.bbc.co.uk/news/uk-england-m ... e-68273459

Very poor, DC electric trains are a mature and well understood technology, Providing power from batteries instead of from a live rail should be relatively simple.
I wonder if this is "new train disease" rather than being directly related to the battery power source. Over complex design, with many opportunities for "computer says no"