Thomas & Friends KNOCKOFFS

The knockoffs do seem to fit right in as very useful engines.

Exactly. That's the point. When originals are hard or impossible to find, or too expensive (even when sold as "used"), and when there is no chance you'll complete some collection (like mine "Four Little Engines" from Rev. Awdry's book) - there are the knockoffs! And - bam! - there they are, for a few bucks. 
I said it before, I'm saying it again: I'm not endorsing counterfeit goods and manufacturing of knockoffs. But in such cases there is simply no other way out!