Cite for the proposition that the engines all wish to be Really Useful?
That’s so common a thread in the series that I can’t believe you’re asking for a cite. All of the fact cards that come with Thomas toys tout each engine’s desire to be really useful.
In “Thomas’ Train,” Thomas is thrilled that Henry’s illness will give him the chance to be Really Useful by pulling a train instead of just coaches.
Or cite for the proposition that the engines are purchased vs. recruited?
I think it’s fair to say that engines don’t roam the countryside looking for work. They are all employees of the national railroad system.
In “Salty’s Secret,” Salty arrives on the Island of Sodor eager to work near the docks. He’s disappointed to be assigned to the quarry – but there’s no sense he’s been enslaved and forced to work there. He’s simply been assigned a job he’d rather not have. Also note that Salty came from similar “salt air” work on the mainland prior to this assignment.
In “Horrid Lorries,” Sir Topham Hatt hires the three lorries to help with work. Does it make sense that lorries may be rented out voluntarily, but trains are slaves?