What is the tolerance on the railings. How far do they have to be apart from each other and the tolerance from beginning to end?
Have you browsed the Tracks and Gantry documentation? At the end of the Construct the Gantry Structure page, there are examples of a couple different track spacings. Since the gantry main beam is 1500mm long, 1500mm is the maximum track spacing. Narrower tracks can easily be used, as shown in the documentation.