The John Bull was one of the earliest steam locomotives used in the US. (The first steam locomotive in the US, and the first designed and built in the US, was the B&O's Tom Thumb in 1829.) The original John Bull (not the one in this photo) was built in England by Robert Stephenson in 1831, then shipped to the US. It began regular service in 1833 and was retired in 1866. In 1885 the Smithsonian Institution purchased it and it is on display in the National Museum of American History.
In 1981 the Smithsonian was planning the locomotive's 150th birthday. It had no noticeable deterioration so they had the intuition that it might still work. They asked the Hartford Steam Boiler Inspection and Insurance Company to determine if it was operable (you want to be careful with pressurized steam), and they determined that it would be safe at a boiler pressure of 50 psi (reduced from the original 70 psi). The Smithsonian tested it and then ran it on a branch track in the Washington DC area, which then made it the oldest operable steam locomotive in the world.
The Pennsylvania Railroad build an exact replica of it in 1939 and that's what you see here. This replica is also operable.