The steam locomotives at Disneyland aren't really diesel locomotives--although they burn (bio) diesel fuel.
Diesel (or more properly, diesel-electric) locomotives would be familiar to you as the trains you see in real life today--big, boxy, often colorful locomotives that look like this:
The steam locomotives at Disneyland operate on a very different principle than the big fella above.
Diesels can operate efficiently going either forward or backwards, but because of their design, most steam locomotives operate best going forward.
This does not mean steam locomotives cannot go in reverse. They're sort of like your car. Automobiles are designed to go forward 99.9% of the time. But they are also designed to go in reverse when necessary. You just wouldn't want to drive to work in reverse!
So, at the end of the operating day at Disneyland, the engineer moves a large lever in the cab to the "reverse" position, and backs the entire train into the roundhouse. In the morning, they pull out going forward.
Hope this helps.