Steam locomotives were in service from the early 19th century to the 1940's and 50's in some cases due to the Second World War. One exception is union Pacific 844, which was never taken out of service and was put to work pulling excursion trains.

They operate off of pressurized steam run through pistons with connecting rods to turn the driving wheels. Since the pistons both pull and push the connecting rods, the divers are set 90 degrees apart from each other so the force from the pistons is fairly consistent.