The verb rail means to criticize severely. When you rail against increased taxes at a town meeting, you speak openly and loudly about how wrong the increase is and point out the problems it will cause.
Rail can also mean "to complain." When your mom asks you to vacuum the house, you might rail against this chore by saying that you ALWAYS do the vacuuming, that vacuuming is SO hard, and that you hate the stupid vacuum! Rail can also mean to spread negative information about someone in an abusive way. If your "friends" rail against you to everyone in school, they're spreading nasty rumors about you, and you should find new friends!