ClipArt ETC
An image of a 'single truck'. They have four wheels, a single motor on each axle, hold a trolley car. The motors are supported by bars b, which are supported by spring s. To prevent oscillation, extra springs S1 and S2 are added. The wheels are generally 30 to 33 inches in diameter.

Single Truck

An image of a 'single truck'. They have four wheels, a single motor on each axle, hold a trolley car.…