Signals I am implementing an Automatic Block Signal (ABS) setup, sort-of. There are 16 control blocks for the track with a signal mast at each end or 32 polls. In all cases the semaphore on the top of the mast/poll is for the mainline path/track and the bottom semaphore is for a divergent path. The aspect (position) of a semaphore (or color of the signal) reflects the status of the next two blocks of track and any switches before those two blocks. In four cases there is not a divergent path (within two blocks of track) so there is no lower semaphore on the mast.


At the beginning and end of each run, one section of track is designated as the parking lot or "staging yard" with four parking places, one for each train. Each parking place is controlled by a dwarf signal lights when the staging yard is active. When the staging yard is not active the dwarf signal lights are turned off and the section of track is controlled by semaphores at each end.


  • Mainline Signals The 32 mainline signals (colored) are set based on the state of next two control blocks and any switches/turnouts before those blocks of track. For example, the aspect of the single signal near the upper-right depends on the state of the switch to the right (red) and the next two blocks of track (red).


The four red signals on the "X" in the middle treat the crossover (the box) as a switch. For example, the aspect of the signal on the lower-right of the crossover depends on 1) if the crossover is committed to the crossing track, 2) if either of the two blocks of track in green are occupied and 3) the state of the two switches between the "green blocks of track" shown in red. 


Similarly the four yellow signals on the "X" depend on the two switches and the next two blocks of track on the inter-loop. Track layout with mainline Signals

  • Divergent Signals The aspect of 20 divergent signals (colored) are also set based on the state of the next two blocks of track and any switches before those blocks. Note a switch before the first or second block must, by definition, divert the loco to an alternate path. For example, the aspect of the yellow signal (top-center) depends on the state of the three switches to the right (red) and the next two blocks of track (red). Notice that the second switch diverts the loco from the mainline (inter-loop). As above, the aspect of the four red signals on the "X" depend on the crossover. Track layout with divergent Signals
  • Other Divergent Signals There are two sets of four divergent signals that cannot be handled as cleanly as all the other semaphores signals above. Except for these eight signals, an engineer can look at the signal(s) on the mast he is approaching and know the status of the next two blocks of track he wants to travel. Look at the red signal on the left going clockwise as an example. Going through the first block (green) into the two thrown switches all is well until reaching the third switch by the "Red X". Which of the two red blocks is the second block of track depends on the state of the "Red X switch." The four green signals present more uncertainty. Using the "green" signal at the lower-left as an example. Taking the divergent path through the first and second switch we reach the third switch by the question mark. If this switch is thrown the two green blocks are reach. However, if the question mark switch is clear, the blue block is reach and the switch by the stop sign determines which of the two green block is second block. The two semaphores on the mast cannot describe the state of all these paths. Track layout with problem Signals

However, in version #2 a reanalysis of how to move locos to/from the parking lot to their assigned path reveals that the alternate pasts are not used, only the path for a figure-eight is used. In the example of the red signal above, when the train reaches the switch by the "red X" only the path continuing across the "X" is followed. Never the path around the inter oval. Similarly, the example of the green signal above, when the train reaches the switch by the question mark, only the path to the green tracks is followed, never the blue path.


    • This results in the realization that the top signals are for the mainline or oval paths and the lower signals are for the figure-eight paths.


  • Dwarf Staging yard Signal Lights The four dwarf signal lights (green) "control" movement at the beginning and end of a run, when this section of track is the Staging Yard. Each signal controls the exit of a short block of track, a parking place for one of the locos. In the beginning the three aspects of each signal reflects the status of the two blocks to the left of the signal. At the end, when the locos are being parked, the signals control movement within the staging yard and in this case the signal on the left is of course red. When this area is not being used as the staging yard, these four signals are turned off and this section of track is treated as a single block controlled by the (tall) signals at each end of the block. Track layout with dwarf Signals

The following algorithm is use for all 64 signals. There are one to four steps to determine the aspect of a signal.


    1. First set of switches. The state of the switch(s) between the current block and the next block of track is checked or the availability of the crossover. The required state of the switches depends on whether evaluating a mainline (top) or divergent (bottom) signal. 

IF there are no switches or the switch(s) are set as required, 

THEN go to step 2 

ELSE Aspect is RED.

    1. First block. Only one loco can occupy a block at a time. 

IF block is not occupied 

THEN go to step 3 

ELSE Aspect is RED.

    1. Second set of switches. The state of the switches between the first block and the second block is checked (including the crossover). 

IF there are no switches or the switches are set as required 

THEN go to step 4 

ELSE Aspect is YELLOW.

    1. Second block. Again only one loco can occupy a block. 

IF block is not occupied 

THEN Aspect is GREEN 

ELSE Aspect is YELLOW


There is a refinement required for the four pair of adjacent blocks at the ends of the layout and the two pair of blocks crossing the crossover, to avoid a lockup. For example semaphore #1 above, must be RED (not YELLOW) in step #4 if block 15 is occupied and the loco is heading CW. Otherwise, the two locos will face each other across signals #10 & #11 with nowhere to go.