Automatic block control is a group of automatic light semaphores set on a fragment of a railway line, that help improve the traffic and increase the number of trains passing through the line during a certain time. Actually, the automatic block control semaphores are near the same as standard light semaphore, with few exceptions: the semaphore pole is white, and the standard a.b.c. semaphore consists of three lightcells (from top: green, red, and orange). NOTE: usually near the stations, where more sophisticated signal images are required, a.b.c. semaphores can consist even of 4-cells and give also 2-lights signal images.

Basic signal images given by a.b.c. signals are: S1, S2, and S5. These images must be generally interpreted as standard semaphore signals, with few special exceptions:

• when an a.b.c. semaphore gives S1 ('stop') signal, the train should be stopped at the semaphore, than - after undefined time (it can be even 10 seconds in practice) - the train can pass this semaphore if the driver doesn't notice any obstacle on the track that make the ride impossible (like other train ahead). Moreover, the train can pass no faster than 20 km/h, until next a.b.c. semaphore. When the next a.b.c. semaphore gives S1 signal, the above procedure is repeated. If the next a.b.c. semaphore gives any ride-allowance signal, the train can pass with regards to general E1 rules;
• when W18 sign is placed at a.b.c. semaphore pole, such semaphore must be considered as forewarning signal, though no W11 signs before, or W1 sign at the semaphore is present;
• when W22 sign is placed at the a.b.c. semaphore pole, heaviest freight trains can pass this semaphore while giving S1 signal. The sign can be mounted under special Infrastructure Headquaters permission, if the brake-distance after the semaphore has 0,6% grade. The train speed should be no more than 15 km/h, to let the driver stop the train in any moment. NOTE: The rules don't specify what 'heavy train' means.
• very exceptional a.b.c. semaphore signal image is S1a, that means no tunnel entrance under any conditions. S1a signal is given by two red lights (the lower one is blinking) and is set at Warsaw tunnel entrances (that lead to Warszawa Centralna passenger station) mostly to avoid train arrivals in case of fire or any similar danger. S1a signal overrides above statements (like passing S1 signal and slow ride).