Abtus Voidmeter, for detecting rail deflection and indicating number of "cans" of ballast necessary to rectify, tripod type, by Abtus, England, c1970.

Railway track must be regularly tested to make sure that trains can run safely and that any movement of the rail surface is not severe enough to risk derailment. Voidmeters detect this movement of the rail surface. The voidmeter is placed on the ballast and when a train passes over a reading is given to indicate whether the flex in the rail is at a safe level.