LNER Class A4
The Class A4 is a class of streamlined 4-6-2 steam locomotive designed by Nigel Gresley for the London and North Eastern Railway in 1935. Their streamlined design gave them high-speed capability as well as making them instantly recognisable, and one of the class, 4468 "Mallard", holds the world record as the fastest steam locomotive. Thirty-five of the class were built to haul express passenger trains on the East Coast Main Line route from London Kings Cross via York to Newcastle, and later via Newcastle to Edinburgh, Scotland. They remained in service on the East Coast Main Line until the early 1960s when they were replaced by Deltic diesel locomotives. Several A4s saw out their remaining days until 1966 in Scotland, particularly on the Aberdeen - Glasgow express trains, for which they were used to improve the timing from 3.5 to 3 hours.
Pennsylvania Railroad class T1
Due to their complexity relative to other steam locomotive designs, T1s were known to be difficult to maintain. Recommended to a maximum speed of 100 miles per hour (161 km/h), T1s were so powerful that they could easily exceed their designed load and speed limitations, which in turn often caused wear and tear issues. A technician charged with determining the cause of frequent poppet valve failures on the T1s claimed to have observed them being operated at speeds of up to 140 mph (225 km/h) to make up time. The T1 was designed to run reliably at speeds of up to . Although such reports are viewed as dubious, some think T1s regularly exceeded 100 mph (161 km/h), making them among the fastest steam locomotives ever built. The price paid for such speed was higher maintenance costs and increased failures in service.