Bridges: under and over?

Is there anyway to place bridges, or make bridges that units can pass over and under?

no

The WC3 engine, from what I understand, doesnt support that

There is a way to make ceilings...
http://www.wc3campaigns.com/showthread.php?t=44494

I'd imagine the same principle could work. Otherwise, create invisible units with the Waygate ability and position them so that as you hit one side of the bridge you can come out the other.

Perhaps it could be done if you could remove the bridge (and put a fake bridge there that wouldn't be a bridge but actually only a unit with the bridge model), but if pathing is done before the map is run and it cannot be changed during play, then the pass will still be unpassable even after you remove the bridge...

Another way is to make a crossroads and spawn pathing blockers that limit movement to only one direction, and use terrain modifiers to raise the terrain under a fake bridge if a unit decides to go above the bridge (the terrain has to be raised exactly right so that it appears as the unit is walking over the bridge), and restore the terrain back if a unit wants to go under the bridge.

However, all these solutions are quite complex to do and only allow passing either over the bridge or under it at one time, you cannot do both at the same time.