Tim probably shouldn't be surprised to see someone else riding on top of the train, considering how many people with powers are here and willing to show them off, but he's not really used to sharing the roofs of the vehicles he's riding on. He's a little more surprised when he realizes just who he's sharing this roof with. Wouldn't he be faster on his feet?

Or maybe he's just enjoying the ride. It is cool.

"Yeah! It's pretty great!"