Coach wheels

[sredgrave]

I have been slowly (very slowly) building my layout and collection of rolling stock over the last 3 years and have just stumbled across an issue I have not noticed until now. All my coaches have run beautifully until the other night when one kept derailing at two specific places. Upon investigating, it was then i noticed that some of my coaches have different sized wheels to the others, which I think may have been the cause of the derailing. It's probably just luck that I haven't had coaches with different sized wheels coupled together before.

Therefore I guess the correct action is to change the wheels on some of the coaches so they all match. However, which size is the 'standard ' size? I am going to be very untechnical here and say I have some with small wheels and some with larger wheels!

All coaches are from Farish but guess some are a lot older than others, not sure if that is a factor.

Any help or advice would be appreciated. My layout is a preserved line so I do change the rakes about so I wasn't planning on keeping specific sets of coaches together all the time.

[PLD]

All the wheels on a single vehicle should be the same size but they don't have to be the same on every vehicle in the train.

The right size is whatever was intended for that vehicle which  could be any of a range from 6mm - 7.5mm depending on the type.
From your description it is a specific issue confined to one coach so leave the others were alone. Far more likely is an axle out of gauge or something catching stoping a bogie pivoting or if it has pivoting couplings those sticking.