Of the 3,305 Buick Roadmasters built in 1954, this one was ordered specifically for this purpose. It was used at Maywood Park Racetrack in Chicago.
Unlike thoroughbred racing, where the horses start from a fixed gate, the gate in harness racing is mounted to a vehicle that starts the race by pulling away from the line of horses. The gates fold forward on the vehicle as it accelerates and then it turns off the track at the end of the straightaway, while the horses continue around the turn.
here's what that appears like when in use:
you can see a whole gallery of a variety of cars that do this https://umbrella7067.wordpress.com/2013/05/07/photos-of-harness-racing-starting-gates-from-around-the-world/