Train surfing is a way to commute.
Train surfing is similar to freight train hopping but different in that the train surfer rides a commuter, not a freight. Both practices are illegal and very dangerous. And yet still there are those daredevils who engage in this "extreme sport" with great gusto.
For the past hundred years or more, people have clung to the sides of trains. Only in the past thirty years has the practice become more common. Train surfing is cultishly popular in India, Indonesia and South Africa. It is done in other parts of the world as well, such as Thailand, Russia, Czech Republic, Denmark, Australia, Germany, the UK and the US. Pretty much anywhere with a commuter rail system and a sufficient level of poverty will have train surfing.
Train surfing is generally the young man's sport, though, like anything, there are exceptions. In Denmark the first "togsurfer" died in 2007. There are archival photographs of German soldiers clinging to the outside of overcrowded trains in WWII.
Train surfing is a way to get laid.
There are some rather impressive videos of train surfers on youtube. Train surfers will do a number of tricks whilst nonchalantly clinging to their speeding trains: smacking signs with their hands, hanging limp, "running" along platforms whizzing past while gripping the train, riding on top of the train, and so on.
Train surfing is a way to live free.
Attempts to curtail this practice with fines have proven ineffective. Every so often a train surfer will die or lose a limb. In Indonesia, the government has begun hanging large concrete balls over the tracks in random intervals, in order to deter the practice.
Such measures seem to be too little, too late as train surfing has latched onto the international zeitgeist. There is a 2010 short film Surfing Soweto about train surfing in South Africa. Swedish pop artist Stina Nordenstam made a song/video celebrating trainsurfing. And here's a report from last year on a train surfer in on the J train in NYC.
Train surfing is something to do when there's no dope to cop.
Far as I can tell from watching one video on youtube, surfers latch themselves onto a train at the station and get off at their desired stop. Only a fool would do otherwise. Knowing the route and when to duck or when the train enters a tunnel would be important also.
But no matter how confident you are of your ability to ride that death train and live to tell the tale, I strongly discourage you from ever attempting to train surf.