Automatic Signals (ABS)
There are three types of SBL in use on polish railways, these are: two-state, three-state and four-state. Automatic signals are controlled by passage of trains and track circuits. SBL signals can be identified by white mast.
Two-state automatic signals
Two-state signals have a two light head comprised of a red and green light mounted on it. This type of signal is preceded by distant signal placed in braking distance.
Three-state automatic signals
Three-state signals have a three light head comprised of green, red and yellow light mounted on it. This type of signal also works as a distant signal informing the driver about the aspect of next signal. These signals are placed in braking distance between one another. This type of automatic signal is the most common type of automatic signal
four-state automatic signals
Four-state signals are comprised of same lights as three-state signals, the difference lies in the fact, that this type of signal can display a S3 aspect. This aspect informs the driver that two blocks in front of the train are free, and depending of where the signal is, driver might have to reduce the speed to 100 km/h at next signal. Thanks to use of this signal distance between signals can be reduced to half of the braking distance for this line. This type of automatic signal is usually used on high speed lines.
S3 signal on automatic block signals
S3 signal on automatic block signal with W1 sign means, that the home signal of the station is showing a proceed with reduced speed to 40 km/h / 60 km/h or is showing a stop signal.
Same signal on last automatic block signal ( With W18 sign ) means that the home signal is showing proceed with speed reduced to 100 km/h signal ( S6, S7, S8 or S9 signal - always with steady lower orange light and green light bar ) or that the semaphore after the home signal is displaying a stop signal ( S5 signal on semi - automatic signal ).
On three-state signals S3 aspect is shown only on last automatic signal ( with W18 sign )