My layout isn't that old yet. Last winter I picked the whole thing up too. Only thing I could help with is I have washed a dirty car before. I used a very mild soap and water. I cleaned the wheels and axles immediately dried them off. I added oil right away to keep out any rust. Not much water got at the trucks anyways as I was so careful.
My main way of regular cleaning is using a dry paint brush and a vacuum. Even compressed air with a regulator turned down works well. Of course it matters how bad the dirt is!
