The London Tube or London Underground is the greatest train network I have yet seen (though I haven’t yet been to Japan). It’s a pretty clever system of moving many people throughout London, conveniently, and regularly.
For me, trains came every 5 or so minutes, and weren’t too expensive. They were clean, modern and really really full sometimes. The only thing I didn’t like about them, is that you had to know where you were going. Some stations provided walking access to others through tunnels, some platforms required you to travel via other platforms, escalators and stairs. With so many people moving around the tunnels, platforms and various stations, you could do one of two things; follow everyone else (or rather be forcibly moved by the mass streams of people), or get lost.