The loading gauge of a rail system is the limits on the cross-sectional clearance profile allowed, whether on a certain route or the system as a whole. If the cross-section of a locomotive or railroad car, including swing-out on curves, fits within the loading gauge it is guaranteed to have no problem with clearances.
Obviously, it would be ideal to have one loading gauge system-wide, but most railroads were not built by the same company to the same standards, but are the product of several since-defunct organizations.