They were used in the British Railways class 55 "Deltic" trains, which were in use on high speed express services until being replaced by the Inter City 125, or HST. As a locomotive power unit, it was very light.

http://en.wikipedia.org/wiki/Napier_Deltic

The Deltic engine makes an amazing noise, they are really distinctive.

I did some track testing and stress analysis work on the 125s, to find the root cause of some suspension failures, and can vouch that they can actually go much faster than 125!

http://en.wikipedia.org/wiki/Inter_city_125

The Deltics were withdrawn from service in the early 1980's, and of the 22 which were built for BR, 6 survive in various states of preservation.

We are now approaching the point where the HSTs are to be withdrawn from service, and it's very sad to say that the work has not been awarded to the only remaining UK train builder - the work has gone to Hitachi!

While I'm not mad keen on protectionism, it seems like madness that we aren't building our own trains any more.