I'd first come across this activity in Barbara Vine's London Underground novel - King Solomon's Carpet where it was referred to as Tube train sledging. In Vine's novel, one of the kids that does this, lies on the top of a Tube and comes to a fairly sticky end. Yesterday's londonpaper reported that a 20 year old schizophrenic man cost the Underground £94,000 and stranded thousands of rush hour commuters after train surfing. He was spotted riding the back of a Jubilee Line carriage at Baker Street. Fortunately he's now been detained under the Mental Health Act.