Darryl,
On the trainz resource directory (I don't have the link handy from this computer), has links to various tutorials on using 3D modeling programs that are used for content creation.
Bit of a warning though, the learning curve is steep, and you'll need an undetermined amount of patience to succeed. If you do get past the frustration with the programs, the results are very rewarding.
Creating the objects though is only one part of the asset creation process. Some assets require scripts to operate such as signals, stations, interactive industries, etc. This is a whole new thing to learn if you're interested.
I'm trying to deter you from making assets, but only to clue you in on the under carriage in the process.
Good luck.
John