Yeah, I've done it (pl_arroyo_a12c) and seen other maps with it (ghost town I think). To summarize what people have pointed out...
1. put a path node where the cart should be when the elevator starts
2. put the next path node where the cart should be when the elevator stops
3. make sure the path nodes are set to point the cart at the node's vector rather than the direction of motion, and point that vector at the proper direction the bomb should be facing while on the elevator
4. have the bomb capture area turn off when it reaches path node 1, then have the bomb cart stop
5. after a brief delay, tell the elevator to move at the same time you tell the cart to move forward at speed X
6. tell the cart to stop moving when it reaches path node 2
7. turn the capture area back on
then just adjust speed X until the cart and elevator move together.
I suggest adding time to the round timer when it reaches the elevator so you aren't creating a scenario where the timer might reach 0 while the bomb cart is on the elevator... because as a player it would feel really aggravating to lose the match because of a stupid slow elevator