<P> Locomotives of European origin have featured pushbutton control of air horns since the mid-1960s . </P> <P> Current production locomotives from GE Transportation Systems and Electro - Motive Diesel utilize a lever - actuated solenoid valve . </P> <P> On April 27, 2005, the Federal Railroad Administration (FRA), which enforces rail safety regulations, published the final rule on the use of locomotive horns at highway - rail grade crossings . Effective June 24, 2005, the rule requires that locomotive horns be sounded at all public grade crossings at least 15 seconds, but not more than 20 seconds before entering a crossing . This rule applies when the train speed is below 45 mph (70 km / h). At 45 mph or above, trains are still required to sound their horn at the designated location (usually denoted with a whistle post). </P> <P> The pattern for blowing the horn remains two long, one short, and one long . This is to be repeated or prolonged as necessary until the lead locomotive fully occupies the crossing . Locomotive engineers retain the authority to vary this pattern as necessary for crossings in close proximity, and are allowed to sound the horn in emergency situations no matter where the location . </P>

When are trains required to blow their horns