Definition of "treadle"

There is an inconspicuous treadle let into the floor and when you step on it an air-operated door-opening mechanism is automatically activated; [...].

1960 March, G. Freeman Allen, “Europe's most luxurious express - the "Settebello"”, in Trains Illustrated, page 141

As a car enters the parking area the front wheels pass over a treadle, causing a timber barrier to lift and allow the motorist to drive in and park. No money is required at this stage. As the car passes over a second treadle, just beyond the entrance, the barrier falls again.

1962 July, “Talking of Trains: Automatic car park at Harlow”, in Modern Railways, pages 11–12