I have some postwar traincars that have some grease on them.
Any Idea how to clean them without hurting the paint
I have used WD-40 to clean many things and have never had it damage anything. This includes plexiglas.
Test it somewhere before you use it, and you would probably be well advised to keep it off the decals and lettering if you can.
Naptha or lighter fluid is a good cleaner and degreaser, as well. I keep Ronsonol on my work bench, and have never had it soften plastic or remove paint.
Be careful with decals, even with water.
Steve
Adjust the soap/water ratio to match the grease load. It usually takes very little soap to lift grease with very gentle scrubbing. I usually use a soft paintbrush (one inch or larger) to avoid any scrub marks on the paint, and even then I go gently. Blot rinse water off, unless you have de-ionized water, in which case it will dry spot free. The decal warning about water applies to this method. For that reason I have not tried to clean my Sunoco tank cars.