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 sacrilegious dumbass who simps over trains, more often than not, over a specific railroad or locomotive. A foamer is often ridiculously stupid and is really hard to argue with because they are so naive with the science and art behind railroading and only care about the trains they set eyes on.
Rail Enthusiast: "Damn that foamer foaming hard for some normal NS SD70ACe with the number 1111."
Another Rail Enthusiast: "Don't worry, I see foamers flipping out over an awful looking Caltrain f40ph with a shitty horn, and thats a normal for me."