 So we took the events where Alice simultaneously measured the position of the train walls in her reference frame. But the problem is, just because the events are simultaneous in Alice's frame, doesn't mean they're simultaneous in Bob's frame. In fact, they're separated in space by LA, and we know that a separation in space in one frame will correspond to a separation in time in another. So in Alice's frame, what happens is the two events occur simultaneously. We have both measurements at the same time on opposite ends of the train. So in Bob's reference frame, what we'll have is first the front measurement will occur, the train will move, and then the second measurement will occur. So then the distance delta XB between these events in Bob's frame is this, which is clearly bigger than the length of the train in Bob's frame. So we're going to have to bust out some algebra. So let's say the length of the train is LB in Bob's frame. In terms of gamma, V, LB and delta TB, what is delta XB?