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.

At least 900VA capacity

Just being pedantic here, but VA is a power rating, not a capacity rating. A UPS has both a power rating that tells you how much power it can deliver at any given moment and a capacity that tells you for how long it can do so.