Automatic Block Signaling (ABS)

vs.

Absolute-Permissive Block (APB)

Automatic Block Signaling is the most common form of railroad signaling and is the easiest to understand. The sequences shown in the section entitled The General Code of Operating Rules illustrate ABS. This type of signaling is best suited for following movement protection (i.e. trains following trains in the same direction) and to a limited extent, opposing movement protection (i.e. trains approaching other trains head-on). As you can see from the sequences in the figure below a short coming of ABS is introduced when double tracks converge into a single track. In the first sequence you can see that the rightbound train located in block 1m has a clear indication (signal 2RM) to enter the single track area as does the leftbound train located in block 5s (signal 4LS).

 
Now in the next figure you can see that both trains have entered the single track area and are traveling towards each other. The rightbound train has an approach indication (signal 3R) to enter block 3 as does the leftbound train (signal 3L). Can you guess what happens next?!

The big red blotch in the following figure is not an ink smear! Itís a collision of the two trains!!! Neither train received a stop indication prior to entering block 3. When each train entered that block from their respective ends signals 3L and 3R changed to red but alas it was too late!

Absolute-Permissive Block signaling can be thought of as an extension to ABS. The main purpose of APB is to protect those opposing movements. In other words, prevent head-on collisions in bidirectional, single track areas. The following sequences illustrate how APB differs from ABS. The primary feature is known as "tumble down." This describes the behavior of the signals along the single track area. Simply put, the opposing movement signals along the single track are forced to red when a train enters that area from a double-track area. The signals protecting following movements (i.e. same direction of traffic) will operate as standard ABS signals. You can see in the first figure that the leftbound train in block 5s has a clear indication (signal 4LS) to enter the single track area. On the other hand, the rightbound train in block 1s has stop indication (signal 2RS) since the turnout to enter the single track area is thrown against its route. Note that in the following figures you will see the Approach Medium (flashing yellow) aspect. If this feature were not available then those signals would display clear (green) instead.

In the next figure the leftbound train has begun to enter the single track area. Since the train is over the turnout the rightbound signals for the single track have "tumbled down" to red. Even if the turnout near the rightbound train was aligned for it signal 2RS would remain red! The tumble down feature only affects the signals associated with the single track area. As such signal 1mR only needs to show an approach indication. Notice that signals 2RS and 2RM have no mileage marker; remember that means they are absolute signals!

The leftbound train has progressed into block 4. As you can see the signals in the blocks it just left are following the ABS protocol.

The leftbound train has progressed into block 3. Again notice how the previous blocksí signals adhere to ABS. This train has the appropriate clear indications to reverse its direction if necessary.

The leftbound train has progressed into block 2 and faces a clear indication on the double-headed signal (1mL/1sL). The train may enter the turnout and proceed on the straight route.

The leftbound train is now over the turnout. Although signal 2RM does not signal the turnout it will remain red until the leftbound train has cleared the turnout on the left. The rightbound train is still waiting patiently (?!) for its turn on the single track!

The leftbound train has finally cleared through the single track area. With its turnout aligned properly the rightbound train can now enter the single track area. You can imagine that as soon as the rightbound train enters the turnout the leftbound signals for the single track will tumble down!