For someone whose application to work for the railroad was twice rejected, Gary Forbes knows a staggering amount about trains. He can recite the horsepower, fuel capacity, and maximum speed of a locomotive, and he can tell you that the freights that rumble past the San Fernando Valley tract home he shares with his collie, Lassie, feature 80 cars on average, because he has counted them. Forbes, who is 54 but guileless and easily amused, like a big kid, is a Rain Man of sorts. Sitting in his living room, framed by shelves of railroad videos, he recalls a train he saw in Northridge hauling sugar beets. "There were four units," he says. "The engine numbers were 8308, 8567, 6671, and SSW9378. There were 87 cars total. It was heading west at 3:35 in the afternoon."

On Monday? Tuesday?

"I saw that particular train," he says, "on July 3, 1990."

Forbes is what's known in railroad circles as a foamer—a fan so ardent in his love for locomotion that he all but froths at the mouth. The term, coined by Amtrak workers, carries a whiff of disdain. Forbes discovered his life's passion while traveling by rail with his family from L.A. to Vegas at the age of 6. "That's when I got the bug hard," he says. "I was in awe of the power, the weight, the whole shootin' thing." As a teen, he spent hours at the tracks near his house, throwing himself headlong into a pastime that baffled his parents. One of his haunts was a railway station in L.A.'s Chatsworth district. Last September, just north of the station, a Metrolink commuter train collided head-on with a Union Pacific freight train. The Metrolink engineer blew through a red signal, a mistake that cost him his life and those of 24 passengers.

An investigation by the National Transportation Safety Board revealed that 22 seconds before impact the engineer had sent a text message to a 15-year-old boy who frequented Chatsworth station, cozying up to the rail crews. The boy's name is Nick Williams, and a day after the crash he visited the scene with friends to honor the fallen engineer, Robert Sanchez, and agreed to appear on KCBS-TV, where he displayed the text message he had received. The initial reaction from the public was one of disbelief. But after teen foamers posted a glowing tribute to Sanchez on YouTube, anger prevailed. In the hours after the crash, Gary Forbes heard the rescue trucks roar past his house, but he didn't give much thought to the carnage in Chatsworth. It involved a passenger train, he says, and he is a fan of freights.

The United States has many foamer meccas—places like the Folkston Funnel in Georgia, where two rail lines meet before proceeding south into Florida, and Rochelle, Illinois, crossroads of the Union Pacific and BNSF railroads, which offers a "hobo fire pit" for grilling and more than 100 train sightings a day. Because of its sunshine and scenery, Southern California is something of a foamer heaven. Tehachapi Loop and Cajon Pass—two stops atop the list of railfan favorites—are an easy drive from L.A., and just down freeway 5 you'll find Fullerton, called "Foamerton" by the local engineers. On the Saturday before Christmas, 20 or so buffs from the Fullerton Foamers and Foto Society are gathered on a station platform for a holiday party. A guy dressed in denim shorts hunches over a camera on a tripod. Another sits with a laptop, using a program called ATCS (Advanced Train Control Systems), which allows him to reproduce the grid used by dispatchers, pinpointing a train's location. Foamers often use scanners, as well, to monitor dispatchers' exchanges with engineers. On this night, Metrolink's Christmas train, a passenger line tricked out with holiday decorations, is scheduled to make a station stop, and hundreds of parents and kids have come to greet it.