In my experience, trains usually travel on railroad tracks as opposed to just darting out on a bridge from nowhere. Limits of available technology did not permit looking to see if a train was coming before the bomb was let go?
Ask a lone pilot who’s flying at night while monitoring three or move MFDs and trying to acquire his target. Usually, intense focus is maintained during on that MFD and not on the periphery.
Unless, of course, you believe Clinton (and the pilot) had the kind of information and skill to target a specific train at a specific moment at a specific place.