And don't forget to check that all of the axles and all of the cogs are seated fully. Worth doing at this stage is to check that everything runs smoothly by running a level surface or bit of track across the wheels. Pressure on the wheels will keep the cogs in place. Fit the bogie, making sure the chassis contact is properly positioned. Fit the base plate - press down the centre and then work towards the holes - and then try the run test with the loco the right way up. Try it on the track too - but don't overdo the pressure which could dislodge older, stretched rubber tyres. If all is Ok, refit the coupling rods and pins, and try the test again.