In this Roblox development tutorial I make an elevator system with two floors. I tween the door parts and for the cabin I tween the model. In order to reduce the risk of falling though the floor, I tween on a CFrameValue and update the cabin's primary part to the CFrameValue as it's changing position. Things to watch:
If your door tween isn't working, make sure the door parts are facing out (front is facing out).
If your elevator isn't moving, make sure you set the Cabin's primary part to floor.