The experimental Gresley W1 No. 10000 “Hush-Hush” was the only 4-6-4 tender locomotive to run in Britain*. It included a number of experimental features including a marine type water-tube boiler working at 450psi. Although this original design is usually dismissed as a failure due to the revolutionary water-tube boiler, No. 10000 did successfully work a number of high profile trains. It never carried a name, although it was often referred to as the “Hush-Hush” due to the initial secrecy of the project.