I've designed a few freelance locos before; one on paper and some in Lego Digital Designer. I don't know if they would work in real life, but I can explain a few (only because I can't remember all of them, as the LDD models are on my old computer and I really don't like using it.)
I'll try to include better descriptions and pictures later!
The one I drew was a small 2-2-4T stirling single saddle tank engine (I don't even know if those are things in real life!)
I also made a 4-4-4T tank engine with sloped side tanks and a very large boiler and firebox, and a cowcatcher in front. The coal bunker in this one is nearer to the tanks and firebox.
There's also a sort of GWR 14xx and J50 hybrid and a shunter diesel based on Salty, but symmetrical (the distance between the cab and either bufferbeam is the same). Another engine is probably one that definitely won't work in real life I simply called Cyril: he's a 4-4-0 that I really tried to make look pretty, but his cylinders are above his running plate and the side rods reach out too far to be safe in real life.
These engines I just make up as I go along with the drawing/building and are totally for fun.
