The Trix wheel cleaning brush is the best and easiest solution to to keeping the wheels of N gauge locomotives clean.
The brush unit fits over a section of straight track, wherever this is convenient for access, supplying power to the cleaning brushes and keeping both of your hands free to hold the locomotive safely. The locomotive is held over the brushes, allowing the wheels to touch the brushes. Track power through the brushes spins the wheels, quickly and easily cleaning away dirt deposits.
There is no need to invert the locomotive, risking damage to fragile boiler and roof top details.