The following information is about the train shown in the movie.
The U.S. Air Force locomotive involved in the wreck / derailment was a Baldwin S-12. The locomotive's number was #1912 (As seen on a manifest paper in the Portal 2 Interactive trailer). The USAF never had an S-12 numbered 1912. Though locomotives of this type are used by the Air Force. USAF #1864 is a good example.
The Rolling Stock
The rolling stock is a mixture of boxcars (Some were refrigerated), tank cars, intermodal flat cars (With containers), and military mobile command cars. The boxcars were a mix of standard freight type (Like this), and specially designed cars based on Amtrak material handling cars (Like the one the Alien busted out of).
The military mobile command cars (Exterior briefly seen in the first trailer, interior seen in the interactive trailer.) are based on Ex troop sleeper / kitchen cars used in WWII to carry military personnel. The cars in the movie had been converted into mobile command / guard centers. Though some could have been used for their original purpose.
- The potato the train would have taken in real life would have used at least 4 different railroads. Southern Pacific, Union Pacific, Chicago Rock Island & Pacific, and Conrail. (Map from the movie)
- Area 51 has no rail access. The closest railroad is the Union Pacific in Las Vegas. According to the map, everything was trucked north to (What looks to be) Battle Mountain, NV instead.
- Though very rare, a car can derail a train, but not in the catastrophic fashion shown in the movie.