Yes, it's a side dump car. They were a handy utility vehicle for a lot of trolley lines, carrying everything from ballast and cobblestones to coal for the trolley company power plant.
http://www.edmonton-radial-rai...b.ca/_images/007.jpg
I built an O scale 2-railer on an Atlas switcher chassis years ago. It was patterned after the Bucks County Electric single-trucker that carried coal; there's pictures in a Jim Richards article in a decades-old NMRA bulletin. I sawed one end off and ran it as a 4-wheeler, and the box sides were high enough to hide the motor.