I think it was still the 50s when I saw this on Frandsen’s Features, an afternoon LA movie show.  So many great movies, some duds.  This one really affected me but I can’t seem to find it.  Sometimes I think maybe it was a Twilight Zone episode, but her goes.
Main character, a maybe 40yo man, starts seeing a symbol on the forehead of people and they end up getting killed somehow.  Try as he might he can’t prevent it.  A group of school children all have it as they board a train.  Eventually he looks in a mirror and sees it on his own forehead.  I always though the actor was Herbert Lom, but can’t find it.  I would love to solve this personal mystery!  Thanks.