The issue about the empty block is which train moved? Three blocks, A, B,
and C. Trains in blocks A and C. When block B becomes active, which train
moved? The ID that is held by A or C? That's the issue.
Now blocks can be built with whichever sorts of sensors you want to use.
While track circuit is following what the prototype used most, and is used
by many layouts, you can do it with optic sensors too. They may take a bit
more work, some optics only give a pulse and logic has to put them together
to know what sections are occupied. I've not used optic sensors for this so
I'm not sure how easy or not it works.
On the issue of blocks between two towns, here are my views, if using ABS
three block rules (clear - caution - stop).
1. If you only have one block, it is easy on the trains to know if anyone is
coming. You'd see a caution that somebody is in the block beyond (you on
track in town one, other train on track in town two). If either moves over
the turnout, the other end would show stop. All of these presume the two
trains are not protected by a turnout set against them.
2. If you put a signal/block boundary at the halfway point between towns
(total three blocks), then you would see clear from either town while
someone is sitting at the other town. If either moved over the turnout, then
the other town would see a caution. Here the issue is you don't know the
difference if that is a train leaving the other town and coming your way or
the other train is just doing a runaround.
3. If you have four blocks, then either train could do run arounds within
that first block past the turnout, and the signal at the other town would
still be clear.
Key here is this is using ABS signals. They are for protection only and do
not grant rights to use the track. Other forms of dispatching would have to
protect the train movements. Options like APB for when you have more than
one block between towns get you closer to making the movements clear but
there are still edge cases that would cloud this. Only using things like CTC
dispatching or double track with direction of travel ABS can you safely move
by looking at the signals only.
-Ken Cameron, Member JMRI Dev Team
www.jmri.org
www.fingerlakeslivesteamers.org
www.cnymod.org
www.syracusemodelrr.org
Ken Cameron