In Lineardraft, high rail level (HRL) is equal to the coordinate on the Z-axis with the greatest value when considering all rails and tracks associated with a given structure.
For structures with only one track, such as STCs, high rail level will equal the Z coordinate with the greatest value from the associated track. For structures with multiple tracks, such as TTCs and Portals, high rail level will equal the Z coordinate with the greatest value across all the rails in all the tracks.