Mystery Dust in MBB Coach Vestibules

[enterprise11]

Lately I've noticed a strange, chalky dust that seems to pile up in between the buffer plates on the 1500/500 MBB coaches. Any clue as to what this stuff is? Looks almost like sand, but chunkier and more whitish gray in color. At first I thought it was speedy dry or cat litter that coach cleaners used for an oil spill or vomit that they sprinkled around and never swept up, but this stuff seems to only be appearing in MBB vestibules and not other types of coaches.

[BandA]

Is it brake dust? If so, not really safe to breath in.

[enterprise11]

No, it does not appear to be brake dust.

[GU1001]

Hasn't the T been dropping fresh ballast lately? Could be stone dust from the fresh ballast.

[Engineer Spike]

Asbestos