If you need to ask the question, it's going to be difficult; if you know how to use 3d software like Blender, 3D studio, or GMAX, an intermodal wagon easier, than say, a passenger coach.
It builds a house then makes it into a train car so gives you the basics. Products are actually one of the simplest things to make they don't need attachment points etc.
How simple, well the basics are fairly easy but getting the texture files right etc and getting into some of the finer points of normal mapping can be moderately frustrating, the drop out rate amongst people trying is moderately high. Start with something simple like a house, don't attempt a steam loco for your first model and follow the tutorial. Once you've done that then think about the next step.
The other approach is to take someone else's mesh and reskin it. There are many intermodal meshes around and its a lot easier to do this. Do a search for container then choose one you like and that has a license that permits you to reskin. Now you edit the texture file, typically this is a .tga file http://en.wikibooks.org/wiki/Trainz/Basic_Reskinning_for_beginners is useful. I prefer Paintshop Pro get version XI or later, ebay is cheap, but GIMP is free and works fine. If you want more detail enlarge the .tga file by doubling it in both directions. If you have a photograph that's fairly flat on that can be used but you may need to straighten it using PSP or take the fish eye effect out. If its purely for your own use than you don't need a license to reskin it. If you are uploading to the DLS be aware you need copyright on the photo for payware so your own photo works best.
In case you did not deduct it out of the previous posts:
- Making a product is fairly easy
- Making a wagon (or any kind of rolling stock) is about the most difficult there is
- Both require knowledge of a 3D tool
Follow the links given by John to start. If you have no experience with a 3D tool, don't expect to be done in a day with a product and don't expect to be done with a vehicle in less then a few weeks (months to actually make it look good).
Whatever you decide to make: You wont finish it without a fair load of patience.
