As norfolksouthern37 says, a .s file is an MSTS mesh file. This subject is very much frowned on because of piracy and infringement of copyright, so you are very unlikely to get any help in that direction. It is far better to approach the original creator to see if he would supply you with the original mesh.
You would need a good working knowledge of GMax or similar and you would also likely run into problems with texturing, alpha channels and animation. Far better to spend the time learning how to create from scratch.