The Slate Trucks are white open wagons for the Skarloey Railway to carry slate or just being empty for an engine's journey. They are often main antagonists on the narrow-gauge railway in some episodes. Despite being constructed for this one purpose, they often carry other types of freight, such as supplies and cattle.
The first role the Slate Trucks had was in the episode, Running Solo where Skarloey carried a train of these in a few scenes. The trucks also have a non-speaking role in it. The Slate Trucks' next role was in New Beginnings where they pushed Sir Handel down the hill during a rainy day. Rheneas noticed that the purple tank engine was coming down the hill at a tremendous rate but with some help from their crews, the runaway had stopped and the trucks' plan had failed for the first time on the Skarloey Railway. Their next role was in Diesels and Dukes where two of these were on Sir Handel's train and after breaking away, crashed into Rusty at the bottom of the hill before Duke came to rescue him. In Hard Work, they taunted Duncan about his favorite song of "rock and roll" before causing another runaway similar to Sir Handel, but only for the Scottish tank engine to derail and crash into a fence. Rusty had to rescue him and Peter Sam was a little bit early with his passenger train to Crovan's Gate which Duncan later made a rude remark about it.
Andreas was hauling a mine train following his argument with Sir Handel over a passenger being left behind which was then stopped by Duke. At Arlesdale, the tank engine yelled OH, COME ON! to the trucks for trying to hold back against him. After a little giggle, they pushed Andreas straight on and knocking both driver & guard off the train to prevent their use of Andreas' brakes. He told them to stop, but the trucks said Didn't you want us to come along? Another lie!. Andreas said it wasn't his idea before crashing through a buffer stop, landing on a part of grassy terrain and veering onto two tracks of the North West Railway's Mid Sodor terminus. Lily, who was hauling a passenger train managed to stop just in time from colliding with Andreas' boiler. Charles later surveyed the whole rescue operation before speaking severely to Andreas who has now lost a second chance.
- Season 1 - Running Solo (do not speak) and New Beginnings
- Season 2 - Diesels and Dukes (do not speak)
- Season 4 - Hard Work
- Season 2 STMY - Mistrust