Seen raspberry pi mentioned some times, I don’t have one, so maybe I’m wrong, but I don’t think there would be an easy way to power it up on a train for example.
You could fairly easily power it from a USB power bank. At least up until the Raspberry Pi 4. The Pi 5 with its weird 5 V / 5 A power requirement is a different beast. They should have gone with something standard like 9 V / 3 A PD. It might still work ok if you don't power lots of peripherals with it.
When you use a typical 74 Wh ("20000 mAh") power bank, you can expect more than 12 hours of runtime, if your average power draw stays at or below 5 W. Of course you aren't going to do much transcoding with a Pi in any case, but multiple concurrent streams shouldn't be much of an issue.