Comment by namibj

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.