The 3D graphics program used to create everything you see in Trainz (except the layouts) is GMax and is available free at http://www.turbosquid.com/gmax
There are links to all the tutorials available from the Trainz 2004 and 2006 home pages. These tutorials will give you basic grounding in all the skills you will need to create any object in Gmax. Don't forget, while you're there, to take the link to download the Trainz "Gamepack", this will allow you to export your creations into Trainz once you do get a copy of it (until then, your creations will only be 'good' in Gmax). In addition, if you follow the instructions from Turbosquid, there are downloadable tutorials that come 'with' Gmax, itself' and all sorts of information from that website (it has a forum just like this dedicated to 3d modelling in general).
Realistically you need a copy of the program to test the items. The usual method is start with reskins using PaintShop Pro etc. and thus dabbling in the config.txt files then move into GMAX but again you need to be able to test.
Have a look at the following sites. They are aimed at MSTS, but you can get hands on experience using gmax as well as info about other software needed for models. Price range from $0.00 to $3,500.00.