A popular choice among AFOLs for building their own creations, Trains are now a sub-theme of LEGO City.
LEGO Trains were first introduced in 1966 in Set 080 with a blue "L" Gauge track. They have been through various iterations since then, along with different power supplies (4.5V, 12V with powered rails, 9V, RC, Power Functions, and now Powered Up), all using the same gauge track. They are now considered part of the city theme.
The LEGO Group provides a number of sizes of wheels for the trains, and many afols build advanced MOCs based on real trains.