This is in relation to a 00 gauge model, but I thought the fault tracing/finding would be useful to share.
The Hornby class 56. A really good model, with a few silly faults. I have owned, continue to own, and work on loads of them. One thing has always bugged me, the running. I generally strip them right back to the bare bones, silly Hornby pcb dumped, and re wired with new correct colour lighting units. I thought I had found the running issue by removing the large capacitor on the motor terminals.
This week I had a seized drivetrain to sort out, so completely stripped it down. Hornby are known for using a 'concrete' grease, which over time dries and causes the chassis to become stiff, or sometimes seized if not ran very often. So apart it came, and out with the degreaser. Once everything was nice and clean, I decided to check that all of the geartrain was properly free running. It's not. I have discovered that the main drive cogs, which transfer the drive from the motor worm to the wheels, have a tight spot and bind. I checked another chassis...exactly the same. So I'm now thinking I have found the issue. The series of photos show a greased up drivetrain upon disassembly, then cleaned, and then finally marked with paint where the tight spot is. I can't see any visible cracking or gear issues, so this must be down to crap manufacturing tolerances. Going to file the offending teeth down a bit in the hope that this will resuly in a silky smooth running chassis. Only took 5 years to find this!!