From the creator of the online comic XKCD, the related What-if XKCD considers if it might be possible to
stop a locomotive with a BB gun. [2] A rigorous application of physics suggests it's possible, but only under some extreme conditions.
Links:
[1] http://www.library.northwestern.edu/libraries-collections/evanston-campus/transportation-library
[2] http://what-if.xkcd.com/18/
[3] http://www.library.northwestern.edu/related-pages/transportation-humor