A system is disclosed to selectively initiate braking at a rear section of a railway train upon initiation of a selected front braking condition at the front section of the railway vehicle. The system utilizes an air sensor preferably constructed in the form of an insect excluder device to detect release of brake pipe air from an exhaust port upon initiation of braking at the front section of the vehicle. An appropriate signal is produced by the air sensor, thus permitting detector circuitry to detect that front braking has been initiated. A transmitter, typically located in the cab of the locomotive, is electrically connected to the detector circuitry. When the detector circuitry indicates that the selected front braking condition has been initiated, the transmitter transmits a brake initiation signal to a receiver located at the rear of the vehicle. The receiver then actuates pneumatic braking equipment at the rear to initiate rear braking. The air sensor may include a generally rigid insect shield connected to actuate the armature of an electrical switch upon passage of air. Such an air sensor may also be constructed utilizing a deformable insect shield configured to operate as a variable capacitor when air passes through the exhaust port. If connected in a resonant circuit, this variable capacitor may provide a detectable voltage variation which can be used to indicate initiation of front braking.