I have a couple of different ones, and it depends on what you really want out of it and how elaborate you want it to be. It can be as simple as listing each car/engine information to adding photographs.
For me, my information consists of the following columns:
Railroad Name
Type of car/engine
car/engine number
Manufacturer specific details; ie Mfg name, Mfg box number, special identifications, etc
Mfg date
Mfg Original Selling Price
Special notes; ie PS2, TMCC, etc
Purchase date
Purchase costs
Estimated Value
I have a separate sheet for Frt cars, one for Pass cars, one for Engines (I separate steam from diesel on the same sheet) and one for buildings.
You can add more information as you feel it necessary.
Hope this helps.