It’s also called “hotel” power and is provided by the locomotive, but separate from “needed to run” power. A train can run with just air and the physical connection, hotel comes with the big “other cable” connected.

Some private cars do NOT use it and instead have their own generator. In theory you could have one with no lights, etc at all.

I’ve been on an Amtrak where it lost hotel power; nothing but emergency lighting until they got to a station where they could swap the locomotive.

But the train kept running, and the conductor had to walk the entire train announcing stops verbally; with no PA system.

It's from the loco which in the US almost exclusively used electric propulsion, just for capex vs. opex balance sheet gaming reasons mostly (except in and around NYC (tunnels) and some very recent electrification efforts (I think bright line in FL was looking at electrifying some trains? Something recently did and improved performance that way.) sourced from medium speed diesel generators housed in the loco.

Way back in the day of steam heating was via open-cycle steam and electric lighting via generators on passenger car axles with a local battery to keep the lights on while stopped.

Eventually with the end of steam they switched to electric heating and can conveniently siphon off electric lights from that.