When any of the fingers strike a projection and are bent back, they hold their position for measurement by an operator who uses a large steel pantograph on the outside of the car.

This pantograph is connected to a smaller one inside the car that reduces the measurement to a scale of one inch to the foot.

Thus, the railroad is able to obtain an accurate picture of the inside of its tunnels and bridges—a picture that will give minimum clearance protection.

Railroad engineers transfer the readings obtained by the car onto working charts, for the reference of freight agents and loading officials and for future use by the maintenance forces and work crews.”