Coupler height gauge.

The HG87 is a go/no-go height gauge: the knuckle should fit into its groove. Its limits represent a prototype freight coupler centerline of 32.5 to 34.5 inches above the rail. Type F and H interlocking heads need particular care with height and alignment. Keep the UV-resin gauge out of direct UV exposure to help prevent dimensional changes. The tight-fitting guides underneath allow a semi-permanent mounting on a workbench track. For a quick check elsewhere on the layout, offset the gauge slightly so the rails support it outside those guides; this preserves the measuring height and makes it easier to move.