I think it would be cool to have a railroad like Goldenrod. It could connect 2 cities, or be a monorail style system to get around a city. If it was a city connector style then it would need to be 2 rail lines: one for leaving the city, and one for entering the city. This way rail carts don't interfere with each other. The only major problem I can see with this, is pokemon getting onto the track and causing the carts to mess up.