I've often thought about how much extra atmosphere you would be able to put into a layout if you could model weather, like rain or fog. Just think of a misty autumn morning and the sound of a loco moving around on a depot or yard then a set of lights then a loco appear out of the mist. Best you could do on a layout would be a smoke machine but they don't give the right effect and if you stand around in the smoke to long you end up choking. Rain you can get the sound, some puddles, people with umberellas but you don't get the ripples in the puddles water running off roofs.