The act of riding a train in any method except inside the train cars. In other words, it's the equivalent of Ghost riding a train.

Usually the act is performed by hanging onto the back, sides, or top of the train while the train is in motion.

The most common target are passenger trains (due to their ease of getting on and off the train at stations) primarily as an extreme sport.
There are some awesome videos on Youtube of trainsurfing.
by Arthur D. April 30, 2007
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The act of riding the exterior of a railway carriage. The only upside of this activity is the high likelihood of the surfer being squished, thus removing himself from the genepool.
"Another kid died trainsurfing."
"It's just natural selection in action."
by Hierophant June 6, 2007
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