The London Underground is usually refered to as The Underground. In the UK, all trains are heated. All long-distance trains are air conditioned. Most, but not all, short-distance and commuter trains are also air conditioned.

In 2026, air-conditioning in English trains is a standard feature on all long-distance (intercity) and newer regional fleets, but it remains inconsistent on older local lines. Major operators like Avanti West Coast, LNER, and GWR use modern "IET" or "Pendolino" trains that are fully climate-controlled. However, some older "commuter" trains in the north and southeast still rely on hopper windows for ventilation, which can make travel during rare summer heatwaves uncomfortable. A major 2026 milestone is the rollout of the new Piccadilly Line trains in London, which are the first "Deep Tube" trains to feature walk-through air-conditioned carriages—a technical feat achieved by placing the cooling units under the train floor. While the "National Rail" network is now roughly 85% air-conditioned, the "London Underground" remains a work in progress, with older lines like the Central and Northern still lacking cooling. For the best experience, travelers are encouraged to check the "train type" on apps like Trainline, as newer rolling stock is almost always guaranteed to have AC.