From Wikipedia,

In the United States, they [railfanners] can be referred to pejoratively as "foamers". There is a dispute over the origin of this term. Some cite the extensive use of styrofoam to create scenery and landscaping in model railroad building, while others trace its origins to the related term "Foamite" (which stands for "Far Out and Mentally Incompetent Train Enthusiast") or claim it refers to "the notion of foaming-at-the-mouth craziness".

I can confirm that the use of "foamer" is also present in Canada as well. The wikipedia article really makes the word sound derogatory, but in practice (with the people I know that use it) it's really just used as an easy and playful way of saying "railfan".

Train enthusiasts