The only tender locomotive in the United Kingdom was the London and North Eastern Railway (LNER) no. 10000, built in 1930 as an experimental high-pressure compound locomotive with an experimental high-pressure water-tube boiler. It was the only locomotive of the Class W1 and became known as the "Hush-hush locomotive" on account of the great secrecy under which it was built. Its trailing wheels were arranged uniquely. Instead of being in one four-wheel trailing truck, the first pair was a Cartazzi axle, mounted in a rigid frame but still allowed sideways deflection against a centering force, as typical of the LNER's practice on its Pacific locomotives. The second pair was in a two-wheel trailing truck.