I've done this tour twice, and probably going for a third time this January. My husband and I both love steam trains, and actually operated an authentic steam train at our local amusement park.
The reason it starts so early is because they take you back to the round house (which is actually square) where they keep the steam trains, there is usually 4 of them but I believe one of the steam trains is up in ohio for a major overhaul. (maybe the Lilly Belle not entirely sure).
They go over the basic mechanics of the steam trains, how they work and if nothing has changed in the last 2-3 years you also get to take a look in the cab of a cold engine. (one that isn't fired).
They go through the starting up of the firing process, firing if you are not familiar with the term literally refers to building the fire in the firebox of the engine. Its the fire that heats the water, that creates the steam that moves the engine. They go a little in depth on that, and it since it is a timely process you usually leave after they get the process started. If I remember right they then take you a train ride back up around the park and explain how the block system works, basically how they keep the trains separated, and the basics of operating the engines and trains at the park.
Afterwards they also spend quite a bit of time on the history of the specific WDW engines, where they came from originally, how they were restored, and generally the history of Disney and trains. Walt Disney if you are not aware was a HUGE steam train fan which is why there are steam trains at both
Disneyland and Magic Kingdom in Florida.
If you really like trains I might also recommend a stop at the Iron Spike room at the Villas at Wilderness Lodge. They have a few of the cars from Walt Disney's original Carolwood Pacific line that he ran at his own personal home. This was partially what spured the development of Disneyland.
Hope this helps! It is a very enjoyable tour.
Jennifer