I’ve been working on my coaches and wagons to improve how they run. There are two things that seem to deliver big improvements.
In many of my models I used the plastic wheels (and plastic axles) supplied with the kit. Replacing these with metal wheels and metal axles improves running. The N Gauge wheels from Graham Farish are very nice. I use a back to back wheel gauge from Markits to check the wheels are the correct distance apart. Many thanks to Richard Doe for recommending these wheels.
A small amount of lead to add weight to the wagons and coaches helps running and stability. I used sheet lead, about 2mm thick, originally for roof flashings. It is easy work. Cut the sheet to size with a pen knife and glue in place with superglue. I attached it to the underside of the wagon or coach, between the wheels, to keep the centre of gravity as low as possible.
There’s more to do to achieve perfect running, but this is a good start.