Newton's third law of motion. For every action there is an equal and opposite reaction. If you push an object backwards, then that object will push you forward. When it comes to space travel we use the exhaust from the rocket. The act of pushing gas out the back of the ship pushes the ship forward. You can actually see this effect for yourself. Stand on a skateboard and throw a heavy object. You will accelerate in the opposite direction.

The big problem is that it's all freight. Our freight lines work well and are actually pretty nice. A little old, but it's still a fully functioning rail system. When it comes to moving people though, we just pretend the U,S can't build rail.