Cruising in the subway offers several distinct advantages. First, it's a confined space, meaning there's little else to do but check each other out. Second, and perhaps most importantly, the subway windows turn into mirrors while the train moves through the tunnel between stations. The etiquette of picking up in a subway car is very distinct, and the worst thing you can possibly do is to be caught looking at someone. Now you're free to pursue him on the basis that he started it.
Some cities are better for cruising than others.
The Los Angeles subway, with barely 11 miles of track, is hardly any good for picking up men at all. San Francisco's BART, on the other hand, might as well start offering sleeping cars for all the action it sees. In London, there's an enduring urban myth that all gay men travel in the last carriage of the Underground, a hold-over from the days when homosexuality was illegal and the only place you could be assured of having more than a handful of fags in the room at once was the House of Lords. And in Paris, the rubber wheels of the subway cars combined with the relaxed French attitude towards sexuality can make for a very smooth ride indeed. The very best subway in the world for cruising boys is probably in New York.