Can I give some advice. If your on a 377 and your power is running low, dont be tempted to plug in to the sockets in the vestibule, marked NOT FOR PUBLIC USE. The reason, these sockets are deisgned for use when the train is at a standstill in the depot, ie for henry hoover. On the move the voltage, amps etc goes up and down and will destroy your charger, laptop etc. Many a time I've come across chargers plugged in and they were burning hot.
Sockets for public use are voltage regulated. We are also limited on how many we can put in on a third rail system due to poor current draw. Esp on icy rails our trains are having issues trying to get a stable draw to run the trains, never mind a load of laptops and phones.
SJ