Not being much into American Flyer trains (I started getting the bug about this time last year, but it didn't take), I am not overly familiar with the 925 tank car. But the picture shows a screw that appears to hold the tank onto the frame. If that's the only thing that keeps it attached (with perhaps a pair of nubs to keep it in line with the frame), is there any reason not to remove the tank before going after the white stuff on the trucks?
I had a Lionel 6464 boxcar that had that crud growing on the trucks. If I recall correctly, the advice I received was to use white vinegar on it. I would imagine that could damage the finish on the tank as well if the silver paint they used is as "strong" as the paint Lionel used.
Hope this helps,
J White