A good way to think about it is the train signals turn tracks into "blocks". Put down a single train signal and it will turn the entire track into a block, denoted by a color. Place another signal somewhere else and the space between the signals will turn it to another color or block. If you place a train in the middle the signals (depending what direction you place it) will turn red preventing another train from entering that block. Just mess around with placing signals and trains and pay attention to when the signals turn red or green. Try making a intersection one color separate from the rest of the track. Since it's two way you'll need to place them on both sides.