Trains not having traction and signal power in Europe is incredibly rare. The railway is essentially its own grid due to the size, so can route power around many local problems. It's obviously one of the last things to be disconnected if there's brownouts.

https://www.raildeliverygroup.com/about-us/publications/acop... §6.1.4 says the UK rail network "never" lost power supply up to 2017, but...

Two years ago, a 45 minute cut to railway power in South East England led to a £10 million fine to the power companies: https://www.theguardian.com/business/2020/jan/03/three-energ...

The PDF also points out that a diesel backup is generally useless, since the line is blocked by the stuck electric trains.