Hi All,
Funny you should ask as I was just on the phone earlier today with a lovely lady who works for the WPYR and she gave me the scoop...
The Steam Train would be about as close as you would be able to get to traveling the Yukon trail EXACTLY as the Sourdoughs did during the Alaskan Gold Rush in 1898 (when they started construction on the railroad, construction was completed in 1900).
Another thing to consider is that the Steam Train only runs on Fridays and only has one departure time - 12:00 PM. The Disney Excursion is an exclusive experience as you will be in separate cars that were chartered by
DCL just for their passengers. So, if part of your party books directly with the railroad and part of your party books through DCL there will not only be a difference in the price paid for the excursion but you will also ride in different cars. Oh, and if you want to ride with certain folks or want your group or family to all be in the same car you need to arrive together.
The Diesel Train runs twice per day, 8:15 AM and 12:45 PM. I does indeed run on the same track as the steam engine so the sights from the car will be very similar. That said, if you choose the non-steam train option, and if you choose to take the earlier departure, you might have time to do more or see more while you are in the port.
And, if you ask at the train depot they can tell you where you can see the Steam Train from the road to get pictures.
My DH and I are currently planning to book the earliest of the train excursions (even though he would love to ride the steam engine) because we want to leave time to rent a car and drive the pass as well. It will cost about the same amount for us to rent the car and ride the train as it would to book just the steam train alone.
if you call the WPYR directly, there might be some direct bookings available for the Steam Train (at $155/Adult $77/Child) but I can tell you that for the time we will be there they have no direct booking tickets available.
Apparently DCL has reserved them all

On the down side, booking through DCL will cost more than booking straight through the line would, on the up side, you do get a nice momento of your trip and since we won't be the only cruise ship in town that day, what with the seats on this train being so limited, there is a good chance that if DCL hadn't reserved seats to make the excursion available for its passengers then the DCL passengers might not be able to get seats at all.
No matter which train you choose to ride upon, I am quite confident that having the chance to ride this line would fall squarely under the "Once In A Lifetime Adventure" category.
I hope that helps. Take care all, help lots of people and have a "Wonder"ful day!
Tisza