I see a lot of the issue with separate detection at turnouts depends on what
are the goals of the layout and signaling. Next you get into the track plan
itself, that has quite a bit of impact too.
The issues get into where does a train need to stop and where are the
signals (virtual or physical) that would tell it to stop. Many times from a
signaling view, making an interlocking of multiple turnouts can save signals
and detectors at the expense of you must stop outside of the interlocking
and can't make changes within the interlocking. Granted that's really part
of the definition of an interlocking, a set of trackwork that is controlled
as one unit.
The best reason for turnout detection is to lock the turnout if it is
occupied. Keeps the route from changing when the turnout is in use. Granted
that can mean more manual lifting if somebody runs a turnout and gets part
of the train scattered across it. I may have been easier to operate the
turnout and not have to lift as many cars (provided they stayed on the track
over the points).
The next big reason for having the turnout as a separate detection is for
knowing traffic direction and telling a dispatcher that somebody either
arrived or left somewhere. If all the track (turnout-mainline-turnout)
between towns are only one block, which train moved? The one at the left or
right town?
In the model here is a case showing some advantages and disadvantages of
interlocking multiple turnouts.:
Single track feeds into two left hand turnouts. Normal leads to mainline,
diverging first turnout and normal second turnout leads to passing siding,
both turnouts diverging leads to yard tracks. The signals are on a three
track bridge over the middle of the turnouts. Having these two as one
interlocking leads to a train on the mainline can stop closer to the first
turnout than a train on the passing that must stop short of the second
turnout. Many model crews have a hard time doing that. Since the second
turnout is normal, they see it as straight track and pull onto that turnout.
But as a single interlocking, they have now fowled themselves. Even using
yellow marked ties to remind crews where the gaps are, they seem to forget a
lot, then complain to the dispatcher why they are sitting at a red signal
(which they caused). At one layout this is three turnouts in a row where the
second turnout is an industry spur and the third is the passing or yard. The
distance difference depending on which track you are on is rather long.
But if you have single track between two points with passing at both, it is
really simple with the turnouts and the single track being one block. They
get a red when somebody is in between and a yellow if somebody is on the
track in the next town. If those turnouts are separate detection, it takes
more logic to manage this same behavior.
Lots more examples and discussions available. Chew on some of this a bit and
give more thought. If you want to do proper CTC, you need the separate
blocks for the turnouts. Otherwise a number of the board operations will not
be prototypical.
-Ken Cameron, Member JMRI Dev Team
www.jmri.org
www.fingerlakeslivesteamers.org
www.cnymod.org
www.syracusemodelrr.org
Ken Cameron