You make your elevator platform and assign it to a func_train. Set the speed, sounds and everything, then tell is to go to a first stop. Set it to whatever. Then, make a path_corner, put exactly in the centre of the func_train. And name it whatever you set the first stop to. Then, move the func_train up to the point you want it to stop at. Put a path_corner here and name it to something, then set the next stop target of the first path_corner to the name of the second one. Name the func_train to whatever you like. Then, make a button or a trigger of some kind which targets the func_train. Experiment with that and you will be a master of elevators in no time.