What will lower your momentum? Once you're going you won't stop unless something stops you. The argument that "if you run out you're dead in the water" is wrong, it should be "if you run out you have no way of adjusting your velocity"
Unable to adjust velocity basically means you're screwed. You're stuck at a constant speed with no way of slowing down or speeding up. And you are definitely not going to be going very fast using a mass driver unless you have a GIANT mass driver attached.
-snip-
Apologies, I took your italics to mean sarcasm, as in "Dude, your bridge is made out of glass. If you get hit there, you're totally forgeted." Once again I am smited by being unable to deduce tone from text XD Sorry!