I think train tracks and different types of trains would be nice. Like you can make your own track like how the water slides are in pieces, the tracks could be the same. While there could be different styles of train cars, one that's more modern like maybe a bullet train, one that's more like a trolley, the traditional looking train style, and a subway train. And each train style can have maybe 3 to 4 cabin styles besides a front end and a back end, maybe even electric trains and rails could be added so that city builds could have more realistic working trains. And the trains themselves could be maybe 2 to 3 squares in width? so that they can fit on plots easier and there could be extra decorations relating to railroads and trains added as well.