Slate Trucks are the Skarloey Railway's main type of truck. Besides carrying slate, they have been known to carry broken branches and sheep. They dislike Sir Handel because he often bumps them. Peter Sam never bumps the trucks though, only if they misbehave.
Once, the slate trucks mistaken Peter Sam for Sir Handel, and decide to play a trick on him. At the slate mine, they snap their chain attached to the winch and run into him, cracking his funnel and denting his boiler.
In the fifth season, some empty slate trucks jammed the winch on the incline and ran down the slope into the ravine, causing an avalanche which buried Skarloey. Another time, they broke away from Peter Sam's train, and crashed into ravine on the old iron bridge.
In the sixth season, Rusty warned Duncan to be careful with the trucks after bumping them. Unfortunately, Duncan got his comeuppance, when he jammed the incline winch and crashed into a muddy swamp.