Imagine we built a railway line in space, ok let's think more along the lines of a magnetic monorail. You have a start point and an end point. The start point and end point are 5 lightyears in distance. You sit on a rail carriage on the mono-spacerail and it's powered by let's say a huge fusion reactor or maybe a solar dyson sphere around a star. Power is not a problem. How fast theoretically could that carriage reach on that rail? would there even be a limit? would a point come as it got faster where it's mass increase would make it impossible to go any faster?