My son got a Magic Tracks set, which is a little battery powered car and some really nifty serpentine latching track. I made him an overpass.
I had originally envisioned connecting the ramps and the overpass together, and adding pegs to hold the track and stop it from slipping, but the slipping wasn't really an issue, and my son asked me to leave it like it is.
It should be compatible with the other Magic Tracks-like sets. (Make sure to check Thingiverse for more things to add to your Magic Tracks!)
Printed on a PrintrBot Plus. I printed the overpass on its side, and the ramps in their regular orientation, with extremely low infill, and supports.