As you may have read, approach signals are visually indistinguishable from automatic signals, but, like home signals , are controlled by an interlocking tower . You may wonder why they exist at all. The following diagram and explanation should motivate the need for approach signals. In this diagram, the lines represent tracks, and the little circles with numbers represent signals.
Trains go from the left to the right.
These signals are roughly 200' apart. Only relevant signals are shown. The colors represent the aspects of each head of each signal. The switch is set for Track 2. As the switch is set for Track 2, trains may not enter from track 1, lest they be derailed, damage the switch, or both, or even possibly collide with a train coming in from track 2.