Abstract: A trip cock device including a pilot valve actuable by a trip lever, when the train on which the trip cock is mounted passes over a trip arm secured to the track, for causing a release valve in the trip cock to be opened and opening brake pipe pressure to atmosphere, at an emergency rate, for effecting an emergency brake application. Axial displacement of the pilot valve, along with a stem thereon, causes a groove in the stem to be engaged by a detent member for securing the trip cock in open disposition until the detent is released manually.