LNER Class A4 4468 Mallard
London and North Eastern Railway locomotive numbered 4468 "Mallard" is a Class A4 4-6-2 Pacific steam locomotive built at Doncaster Works, England in 1938. It is historically significant as the holder of the world speed record for steam locomotives at .
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.