I keep an inventory of all of my trains, including some that I've sold to others. These are cars that are lettered for my own free-lanced railroads, as I don't want to make duplicate numbers when I build or paint more rolling stock. For instance, I sold off a lot of '50s, '60s, and '70s equipment when I decided to backdate my layout to the '30s. Most of my steel boxcars are numbered in the 7800 series, a few of which were sold, along with more modern boxcars in the 77-and 7900 series. Unless someone has obtained some of my custom lettering, there are no duplicate numbered cars floating around out there.
All of the prototype cars that I currently own are listed alphabetically by railroad, and sorted by car type and numerical order. Information includes the car length, the model's manufacturer, and whether or not the car has been modified (brake gear, wire grabs, etc.)
The same goes for my free-lanced cars, with the added notation if a car has been sold, and to whom, if known.
I also keep an inventory of locomotives, by roadname, and number, along with manufacturer and any important modifications, such as remotoring, regearing, etc. In addition, I keep a maintenance log for all locos, covering repairs and modifications, and scheduled maintenance.
Maintenance-of-way and wreck equipment gets the same treatment, along with an abbreviated "history". For instance, I have several downgraded and renumbered freight cars in work service: these have a notation as to their former number when in revenue service.
Every car on the layout, including scratchbuilt and loose cars bought from the bargain table at the LHS has its own box, stored under the layout. If I go to that big roundhouse in the sky, or more likely to the fireman's ball in the other direction, my grieving

family merely has to match the reporting marks and car number on the box end to any cars that are on the layout. (Many cars are not in their original boxes, especially scratchbuilt cars or those bought loose at bargain tables or garage sales) When I do my next update on the inventory lists, I intend to set a value for each car, based on what I know they'll sell for, 'cause sell 'em they will.:cry:
In general, the car manufacturer is really only important in the instances noted above. For my own stuff, almost all of it has been custom painted, altered, or renumbered, so its value is dependent almost solely on whether or not I did a good job on the modifications.
Wayne