a steam engine would be cool. then i can recreate the key scene in back to teh future 3:

"marty you have to learn to think four dimensional. there is nob ridge now, but there will be in 1985.
the train pushes the delorean over the track, until it reaches 88miles per hour. we get transported back to the future and glide over the now finnished bridge."

ißd like that :-p