More like, going again - the Big Boys were taken out of service in the late 1950's, early 1960's.

This one was sitting in a train museum in California for 50 years before the Union Pacific got it back; they them spent several years restoring it to running condition.

The UP has a some information about this train on their website: https://www.up.com/heritage/steam/4014/index.htm

Talyllyn on its eponymous railway in Wales has been in regular service since 1864. The big boys are actually some if the youngest steam engines, built right at the very end of the age of steam in the US.