InterCity 125

The InterCity 125 was the brand name of British Rail's High Speed Train (HST) fleet, introduced in 1976. The InterCity 125 train is made up of two power cars, one at each end of a fixed formation of Mark 3 carriages, and has a top speed of 125 mph (201 km/h), making it the fastest diesel-powered train in regular service in the world at the time of its introduction and for many years afterwards. Initially the sets were classified as Classes 253 and 254. A variant of the power cars operates in Australia as part of the XPT.

After three decades, the majority of the HST fleet is still in front-line revenue service under privatisation, and while the InterCity 125 brand name is rarely mentioned officially by the private train-operating companies (TOCs), the HST still forms the backbone of express services on several British main lines. Most are expected to be replaced within the next 10 years by the Intercity Express Programme, but a number will continue in use on London to Devon/Cornwall services, where there are no plans to electrify the lines. Engineers from the companies responsible have calculated that, with a certain amount of rewiring, the Mark 3 carriages used can be made to last until at least 2035.

The power cars now have new engines, and the coaches have been refurbished. The trains currently operate between London and Penzance, Plymouth, Newquay, Paignton, Exeter, Cardiff, Swansea, Carmarthen, Pembroke Dock, Edinburgh, Aberdeen, Bristol, York, Inverness, Harrogate, Hull, Bradford, Nottingham, Sunderland, Leeds, Great Malvern, Hereford, Glasgow and Cheltenham. One has also seen departmental use, as Network Rail's New Measurement Train, converted and in departmental use since 2003.