I've always been more of a light rail / subway type person myself, and I LOVE riding subways in big cities. A few bucks for a day pass, and I'll be happy all day. Here's some of the most frequent subways I've been on: the Washington DC Metro-rail...a very fast, quiet modern subway system with unique architecture. It's beginning to show a little age, and lack of maintaince is starting to catch up with it. New York's subway system is the polar opposite. A fascinating, traditional subway system that reveals a tremendous amount of history. It's unique 4 track system can be complex and confusing. NYC's subway is recovering from years of neglet to being quite a nice well-oiled machine, and a beautiful piece of history. Atlanta's Marta is another rail system I've been on. It is sort of modeled after DC's metrorail, but is hardly anywhere near as big and sophisticated. It too, is suffering from deferred maintaince, as well as valdalism problems. It's a good design, but sadly, it is not quite large enough yet to really alleviate traffic issues. Still, despite its shortcomings, it is a nice example of a modern commuter rail, with PWM controllers, electronic switching, etc.
One of the most interesting things that always puzzled me that I just recently figured out was why the shape of the subway rail cars was different. I figured out it has to do with the way the tunnels are made! Washington's metro-rail cars have sloped sides to their rail cars, that are angled in. New York's are perfectly straight. It's the tunnel shape that dictates the shape of the cars. New York's subway was built using "cut and cover" where massive holes were dug out of the city streets, and the subway tunnels were built using ordinary steel beam construction techniques of 1903. The shape of the tunnels was basically square. DC's tunnels however were dug out using a tunnel boring machine deep below the city's streets (WMATA also contains some of the world's longest escalators!) the tunnels are round. A flat bottom was created to mount the tracks to. The shape of the train cars reflects the maximum use of the tunnel shape as the rail cars travel through them!