Wanting to make my own locomotives... hopefully. Any advice?

Hey, so I had the game for about half a year thanks to my girlfriend being the absolute boss lady she is buying me it for Christmas ( her original gift was going to come to me late so she bought me the game to "make it up to me". She's so precious I love her so much)

Anyway, I always threw the idea of making my own locomotives for private use or even to release publicly. Any advice on how to do so?