A traditional gantry machine's crossrail and milling ram is mounted to columns with the crossrail and columns traveling together on a set of guideways at floor level. A high rail gantry replaces the moving columns with fixed walls that have the guideways located on top. This significantly reduces the moving mass resulting in increased contour accuracy as well as providing the highest dynamic behavior.
The benefits to this are increased accuracy, better dynamic behavior and faster acceleration and speed. The downside is that an adjustable crossrail is not available for adjusting to different height parts. Also due to the walls, part loading/unloading is more difficult.