Yeah, per sibling I think this is a joke about building a double decker train that's not tall enough for people to stand up in.

I think they're joking. A double decker train is only feasible in the UK if everyone sits on the floor.

Because of the tight loading gauge in Britain, trying to cram two decks in would make it very small. It's been tried (once[0]) and they weren't able to make it fully double-decker, quick to load/unload, or especially comfortable.

I agree that they're fine in countries with larger bridges and tunnels -- Amtrak's Superliners are palatial in size -- but not for us. (Except probably for the Channel Tunnel rail link, which is built to French gauge).

[0]: https://en.wikipedia.org/wiki/SR_Class_4DD