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Diagram 177: Block Diagram of a Circular Layout
It can be assumed that the physical wiring of the layout displayed above is done in a
way that the track power will not change its polarity when a train cycles around the
loop. In other words: the physical polarity of all blocks in the above diagram can be assumed
to be identical.
The situation in TrainController is different. TrainController does not want to
rely on the fact, that the layout has been wired in a certain way. Additionally, the structure
of many layouts is much more complicated. It can contain reversing loops or several
levels, it can be based on a modular structure, etc.
For this reason TrainController uses the logical polarity scheme described above. If
the layout displayed above is wired in a way that the track power will not change its polarity
when a train cycles around the loop, then the train will pass “Hidden Yard 1” and
“Southtown” to different logical directions (left or right), even though the physical polarity
of the track power remains unchanged. A train that passes “Hidden Yard” to the
right at positive track polarity will pass “Southtown 2” to the left at the same track polarity.
As a consequence “Hidden Yard 1” and “Southtown 2” have different logical polarity
from the point of view of the software. The differences with regard to the logical
polarity of the particular blocks are marked with a yellow or blue arrow in the diagram
displayed above.