The conventional DC cab control system comprises 25 blocks. Each block may be independently assigned to either cab, allowing trains to operate independently, and meet or pass each other. Every block junction is governed by two block signals, one facing each direction. Signals are driven by a comparator which senses voltage and polarity on each side of the block junction. If they are equal, the upcoming block is assigned to your cab, a green (proceed) aspect is shown.

If they are unequal, or both unpowered, the upcoming block is not assigned to your cab, a red (stop) aspect is shown.

Trains attempting to pass a red aspect will stop anyway, as there is no power available to them past that point.