On the workbench recently was this Black 5 which, I was told, didn't work.
It is an interesting loco - a Hornby model that appears to have been tickled by an enthusiast. I think it's a collection of different parts brought together into one model, with a bit of added detail.
The chassis was cast metal and designed to be motorised. I'd say early Hornby or Tri-ang, with home-brewed pickups that didn't touch the backs of the wheels. A bit of work with the soldering iron fixed that. It helps if the pickup wire isn't floating around.
Poking around for electricity, I dropped the tender drive out, and the middle wheels fell off. They didn't seem to be very well attached and wobbled like crazy, but I suspect work after a fashion. The sideframes would hold them on the stub axle, and I don't have the means to bush them properly.
This all improved things a little, but the model was still sticky. Eventually, I worked out that the valve gear "weathering" consisted of painting the metal with a mix of varnish and treacle. Stripping it down as far as I could, then polishing the muck away with a fibrepen, combined with unbending a few links, cured the problem.
Eventually, the model ran reasonably well, proving that nearly anything can be rescued. A couple of hours work and the loco will live to chuff another day.