First find a map with a bit of straight track. the middle of the figure 8 works well

Next delete a few pieces of track

now use the scroll wheel to make the track go down until it looks like mine. then place the track

now go under the ground and you can see your track.

if you want to make the track longer make it go slightly up so its level.

then just keep adding on to your track until its long enough. drive a train on it and you should come out with results like mine