I originaly planned on running 40-50 car trains, but after operating for a while, I have cut the train length down to 25 to 35 cars. Much longer than that, and the train starts going around too many corners and doesn't look right. If you plan on 25-35 cars, and design your layout for that, you can get more towns in your layout. I don't know how big your layout will be, but if your trains are around 10-13 feet long, and you leave at least one train length between towns and sidings, you can see that you will be able to do much more with the room you have, than if your trains are 16-20 feet long. Design your layout good, and put lots of thought into it, so you don't have to redo it, (you will anyway!)
Mike.