The class 370 APT-P, what stands for “Advanced Passenger Train Prototype”, was the second iteration in the APT programme whose goal was to design a British high speed train. After the APT-E that was powered by gas turbines, the three APT-P sets were built for lines electrified with 25 kV AC. They were manufactured between 1977 and 1980 by British Rail Engineering Limited at Derby.

Each set consisted of two central four-axle power cars and two six-car passenger sections which were articulated on Jacobs bogies. Each power car had an output of 3,000 kW or around 4,000 hp. Technically they were designed for a top speed of 155 mph or 250 km/h, but limited to 125 mph or 201 km/h in service. The bodies of the train could tilt by up to nine degrees to allow higher speeds on existing lines.

In the following years, the three sets were used for extensive tests. In December 1979, one set a British rail speed record with 162.2 mph or 261 km/h that was only broken by the Eurostar. In the end the test programme was stopped in 1986. In the meantime, the class 43 High Speed Trains were a greater success. Although these are slower and do not tilt, they were much cheaper and can be used on all non-electrified lines.