The vast majority of passenger cars prior to the present-day type
of 480VAC HEP systems worked on 32 volts DC, provided by big belt
or shaft-driven generators from one of the wheelsets on each car
and a big set of batteries on each car. When the car was stopped
or moving slowly, the batteries powered the lights, etc. Above a
certain speed, the generators took over and recharged the batteries
while also providing power.

It was a very high-maintenance system prone to a lot of issues.
It was OK when there was nothing better available and railroads
had thousands of carmen all over the place to take care of them.