The natural degradation of the NYC medium grey finish differs from most Lionel paint jobs from the same era. Aging produces a waxy, dull texture that, from my experience, doesn't respond well to normal mild cleaning. The dirt binds to the surface like glue. Even though the finish feels tacky, it does seem durable and can handle most harsh liquid cleansers. It's the waxy surface that ends up looking blotchy after the dirt is removed. I found it necessary to polish the finish with a short bristled brush after it was dry to resort an even, satin sheen. As usual, I caution using all but the mildest cleansers on other painted or decalled decorations. Spit works just as good on details in most cases.
Bruce