Hello One and All.
For plastic wheels the previous methods are the best.
But for metal wheels I have my own way.
Set your wagon in a cradle upside down.
Then with a modellers drill with a wire brush attachment,start the machine and apply gently to each wheel,the wheel will rotate,apply a little pressure,and you will find your metal wheels become clean.
Warning,use safety glasses as parts of the brush attachment fly off.
Also do not overdo it as you will wear the wheel down.
I hope this will help.
Regards to all with greetings from Holland.
John (Chips).