Currently underway (June 2024) are trials of a battery - electric train that uses fast charging technology.
To simulate a real-world application this test train shadows the West Ealing - Greenford diesel passenger service, stopping at stations etc but it is not carrying passengers.
The train re-uses former London Underground District line D78 stock - the District line being one of the London Transport services which dates from the steam train era and uses subsurface rolling stock which is approximately the same size as main line trains. (This means that it is NOT a 'tube train').
More About Fast Charging
The reason for the trial is to see if battery technology can offer a viable alternative solution for services on lightly used routes which (to reduce air pollution and generally make more attractive to passengers) would benefit from electrification but for which the traditional forms of overhead wire or third rail power supply systems will not be financially viable
The fast charging issue is important because traditionally the weight and daily travel range of the batteries has proven to be significant drawbacks. Fast charge battery systems have already been found successful on some bus and tram / streetcar / light rail networks, The benefits of fast charge technology include:
*carrying fewer batteries (hence less weight)
*the ability to extend the daily range - instead of relying on one overnight charge.
It is usual to carry out fast charging during layovers when the vehicle is stationary - such as at the route terminus. The bus industry coined the phrase 'opportunity charging' which derives from the ability to give the batteries a top-up charge where and when the opportunity arises ... all that is needed is a battery charging facility.
Note that this is not the only form of public transport battery charging - some railway services use trains that can collect power from the fixed infrastructure (typically overhead wires) where these are available and travel in battery mode when on non-electrified routes. This mimics a trolleybus technology that is used in many towns and cities overseas called 'in-motion charging'