[3DWolf]

I wouldn't advise using a 45 - the force required is nuts and if that device fails it could potentially hurt someone. We thought about a garage door spring as well but those are dangerous - people have gotten cut badly by them.
60 is a little steep but you don't need to spend so much energy in the horizontal if you fire your device while moving forward. You only need to get the ball up 6.5 + a little, you don't have to clear the entire overpass.
Someone on here in another thread calculated an estimated 44.7% energy recovery on impact for a trackball. That's just in estimate - but the ball should in theory bounce off the overpass rails if you get it up there.

Don't take my words as your 100% valid source - the majority of the time I'm wrong. But in seeing my wrongs I learn.