Whilst commonly used in continental Europe and North America, the restrictive railway loading gauge in the United Kingdom prohibits normal double-decker trains with two fully separated decks.

As of early 2026, Britain does not currently have double-decker trains in regular passenger service, though a major shift is on the horizon. The UK's historic railway infrastructure was built with a "loading gauge" (the height and width of tunnels and bridges) that is generally too small to accommodate standard two-story carriages. However, in late 2025, Eurostar announced an order for the "Celestia" fleet, which will be the first high-speed double-decker trains to run into London St. Pancras. These trains are expected to begin operations in 2031, taking advantage of the specially built High Speed 1 (HS1) track which was designed to European larger-gauge standards. Historically, the UK experimented with a "split-level" commuter train in the 1940s on the Southern Railway, but the design was cramped and ultimately abandoned. While you can find double-decker buses on every corner in London, the nation's rail network remains a single-deck system for now, with the upcoming Eurostar expansion serving as the only exception to this long-standing geographic constraint.