> Every motor vehicle, when operated upon a highway, shall be equipped with a service brake and an adequate horn or other warning device which is capable of emitting sound audible under normal conditions from a distance of not less than 200 feet, but no horn or other warning device shall emit an unreasonably loud or harsh sound or a whistle. The driver of a motor vehicle shall when reasonably necessary to insure safe operation of the vehicle give audible warning with his horn or other warning device.
Additionally, California Vehicle Code section 23110 prohibits the modification of any vehicle's exhaust system to amplify or increase the noise emitted by the vehicle.
Therefore, it is generally not legal to use a car-mounted train horn in California, unless it is installed and used in a manner that does not violate these prohibitions.
