If one looks at the observation car, you'll see that the car sits way too low to the truck underneath it; you can also look to the right side of the "car" and it becomes pretty apparent that the "cars" are not real in any way; they are simply decorative.

If you climb up to the locomotive window, the cab is pretty well intact as well as the access panels to the electrical panel. However I've tried and have not been able to get inside the cab. (I've had more luck getting in the cab of the steam locomotives across the street!)

I haven't heard that a final decision has been made to the train during the remake of the plaza; however the Hollywood Pictures Backlot is getting a replica Red Car as an attraction. So Disneyland/DCA will now have three different trolleys - the Main Street Trolley, the Toontown Trolley (no longer a ride, but a static display) and the Hollywood Red Car.

I've always wondered the choice of the Western Pacific train at the plaza and not a Santa Fe train - Walt Disney was always a Santa Fe fan, and in the early days of Disneyland the Santa Fe was a corporate sponsor of the park. Plus, the Santa Fe covered more of California than the Western Pacific did (although the Southern Pacific would be even more...but Walt himself didn't seem to care for the Southern Pacific very much.)