Because there is a magnetic force between the train and tracks. This also makes the train more efficient and so there is less friction.
The wheels have flanges on the inside edges, they keep the wheels on the rail. The wheels would not stay on the tracks without the flanges.
Trains run on tracks. The tracks are made up of a pair of parallel rails.
Trains travel on rails ...
Smoke
Tracks
Tracks
Trains that run on tracks.
They are used for trains to ride on them
Trains drive on railroads or tracks.
They use freck'in wheels!
It makes them self guiding.
There are various different types of trains sold by MTH Trains. MTH Trains sells electric toy trains, and train tracks for their trains primarily, though.