The Pennsylvania S1, the longest and heaviest rigid frame reciprocating steam locomotive ever built, was a single experimental locomotive that was revealed at the New York World's Fair in 1939 and was scrapped in 1949.
On September 21, 1939, at the New York World's Fair, Emmett Brown explained the presence of his Jules Verne train by stating that it was going to be part of the railroad pageant with the Pennsylvania S1. After Doc stated this, his son Verne thought that they were going to visit Pennsylvania. However, Doc explained that it was a famous train, and then took his family to see it.
- Back to the Future (IDW Publishing)