1829 Stephenson Rocket

Stephenson's 0-2-2 Rocket of 1829 is probably the most influential steam locomotive, as its innovations remained common features on engines around the world for most of the next 150 years:
-Boiler with multiple fire-tubes
-Firebox separate from boiler
-Blastpipe to force exhaust gases up the smokestack
-Cylinders directly connected to driving wheels

Despite its importance, Rocket's operational history is fairly obscure.  It left the Liverpool and Manchester Railway in 1834, and was used on a railway in Cumberland until 1840.  It was modified while in service: the cylinders were brought to a shallower angle, the smokestack was simplified, the firebox was expanded, and a buffer beam was added up front.  Rocket was donated to the London Science Museum in 1862, and has spent the last 150+ years on display there, welcoming visitors to the gallery on modern technology.