A term railroad employees use to describe a railroad enthusiast / railfan and the railfan community at large. Most often used disparagingly.

When you eat dirt for the first time and leave your family to build a shack in the woods

Orignally used to describe railbuffs, it now describes anyone who gets excited about something so much that they start foaming at the mouth.

Somebody who is obsessed with trains, may or may not work for the railroad. But they love trains so much they foam up in the mouth staring at trains, and jerk off with the foam.

A person who is extremely obsessed with trains, usually they don't stop talking about it.

The type of n00b who would use derogatory rail slang like SCAXliner or Amtrash, complains about the tuning of train horns, tends to endanger his or other's lives for a good photo, and angers railroad crews through his actions.

Also tends to spam youtube haters' channels, and tend to harrangue people via internet, cell phone, etc.

A foamer is a rail fan, most likely someone who has just gotten into the hobby, who foams at the mouth when they see a train.