Posted by: "Don Thomas"thomasd@xxxxxxx donaldmthomas
Date: Tue Nov 26, 2013 10:53 pm ((PST))
Very generally, and not pretending to be running a Rules or signal design class
here: Interlocking signals are used to protect places where tracks meet or
cross. They can be set specifically for each movement by an operator either
locally or remotely, or they can be triggered to operate automatically to clear
a route for each train approaching, blocking any other train arriving on a
different track until the first has passed. The control mechanisms for the
signals and switches are interlocked mechanically, electrically or
electronically in such a way that a signal cannot be cleared unless there is a
safe route through the interlocking zone. This means all switches (if any) must
be set for the train to move through on the intended route, and there are no
other routes cleared which would conflict with it. Once a signal has been
cleared, the switches on the route, and all signals governing potentially
conflicting routes, are locked in place until the train has gone through (or
the signal has been set to STOP and a timer runs out). Not all interlockings
have switches, as they can also be used to govern diamond crossings. But they
must all contain signals. An interlocking can involve a simple diamond
crossing or a junction of two tracks, or it can be a complex beast with a
spaghetti bowl of crossovers. Under Interlocking Rules, a STOP signal is
absolute and trains must not proceed after stopping without explicit
permission. The default state of an interlocking signal is STOP unless cleared
to a less restrictive indication.
Block signals exist to protect trains that are not going through junctions. In
their simplest form they protect trains from running into the rear of a train
ahead of them. Automatic block signals are activated electrically by the
presence of a train, altering an electric circuit flowing through the rails.
(There are manual block signal systems, and other manual block systems, which
are not relevant to your question.) Under ABS STOP AND PROCEED is normally the
most restrictive signals, normally meaning proceed no more than 15 mph and
prepared to stop short of obstruction, opposing train, broken rail etc.
(Remember this is not a rules class.) However ABS also allows absolute STOP
signals. A simple ABS system is unidirectional and is used on a double track
railway, with the current of traffic in opposite directions on the two tracks.
Absolute Permissive Block (APB) signals allow ABS to function in both
directions on a single track without head-on collisions or serious delays. APB
provides absolute signals at the end of sidings or double track. These are set
to STOP when an opposing train enters the other end of the section, forcing a
train to wait there until the opposing train has gone past. Along the single
track the signals are permissive, i.e. STOP AND PROCEED allowing trains in the
same directions to follow each other. These intermediate signals work the same
way as simple ABS signals. The default state of an automatic block signal is
CLEAR unless a train in a nearby block sets it to a more restrictive
indication. Interlocking signals are wired to work like block signals so the
presence of a train will override a less restrictive indication the signal
would otherwise be set to.
Block signals do not convey authority to operate a train. They are overlaid on
the system of written authority used on the railway.
Centralized Traffic Control (CTC) combines some features of interlocking and
block signals. Controlled points such as junctions, siding ends, end of double
track, or crossovers between two or more tracks, are controlled by a dispatcher
or rail traffic controller. These technically are interlockings. The signals on
the main track or tracks between these points are technically automatic block
signals. They are controlled solely by the presence of a train rather than by
the operator. (The controlled signals are also overridden by the presence of a
train on adjoining blocks.) The indications in CTC are consistent with those in
ABS and interlocking rules. The big difference is the rules governing CTC. CTC
signals themselves constitute the authority for train movement within CTC
territory.
For the most part, CTC signals at junctions will look and behave like
interlocking signals, and CTC signals between controlled points will look and
behave like block signals.
Block signals in ABS generally have one or two heads. In ABS a two head signal
will normally have the heads staggered to indicate they are permissive. However
if there were a two head signal at a siding end under APB, the heads would need
to be vertical to indicate it is an absolute signal. Interlocking signals may
have one, two or three heads. Signals protecting diverging routes would
normally have at least two. A single head interlocking signal would need an “A”
sign to show it is absolute, and a two head signal would have to be vertical
rather than staggered to show it is absolute. A three head signal can only be
absolute; it cannot give a permissive STOP AND PROCEED indication. Dwarf
signals can have only one or two heads even within interlocking limits.
As far as what side of the mast the signals are placed, I would suggest you
research prototype practice for the locations you are interested in. The only
restriction is that a two head permissive signal must be staggered, and as far
as I know the higher head must be to the left and the lower head to the right.
As I warned at the outset, this is neither a rules class nor a signals
symposium. People with rules cards and signal specialists can correct me ten
ways from Sunday. I am just offering the quick and dirty layman’s 25 cent tour
of the subject to give you a quick grounding. There are lots of better sources
on line which can give you more insight.
