You should discuss the matter of the title change, from salvage to rebuilt, with the person or dealer that you are buying the car from to see if they first can tell you why there was the change. If they cannot give you the information you are requesting then see if your state's Department of Motor Vehicles can since the title changes would have needed to go through them normally.
Most likely the car was given a salvage title due to the cost and amount of the repairs from the collision with a train. The car was then likely repaired and thus the salvage title was changed into a rebuilt title so that the car would be allowed to be driven on the roadway. States laws differ with regards to how these types of titles are used and issued so again check with the DMV.
For example in Connecticut if an insurance company has determined that a vehicle is a total loss or the word "Salvage" has been stamped on the title, it is illegal to operate this vehicle upon any road in CT. Technically, the registration on this vehicle has been cancelled.
In CT if you rebuild a salvaged vehicle you must re-title the vehicle. The vehicle cannot, however, be re-titled until it passes a Salvage Inspection. Once the vehicle passes the salvage inspection, you will be issued a new title (without the "Salvage" stamp on it) with the words "Rebuilt" printed on it.
Keep in mind that if you do buy this car with the rebuilt title it may be difficult to obtain insurance on it. Due to the previous condition of the car, many insurance companies will not insure a vehicle with a salvage title or if they do, it is only for liability and not for physical damage coverages (collision and comprehensive).