[pmyteh]

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

[ExoticPearTree]

I rode double decker trains in the Netheraland, US, Germany and France and they were very good - same space and I would say headroom as a normal train.

I was not aware that the UK has a different gauge than Europe and US.

[pmyteh]

Yes - to a surprising extent. The best diagram I've seen overlays them[0]. The British gauges are the smaller ones starting with W - with W6 being available essentially everywhere and the higher numbers on specially cleared routes to make it easier to move larger freight containers. GA and GB are standard Western European gauges: both taller and wider.

There's a surprising amount of global variation as much of this stuff wasn't standardised until after most railways were built. AIUI that's even true in the US, where the routes in the West can often take double-stacked containers and Amtrak's Superliners, and further East they often can't.

[0]: https://rfg.org.uk/wp-content/uploads/2017/08/Loading-Gauge....