From: roma@ux1.cso.uiuc.edu (Jon Roma)
Subject: Re: What is an interlocking? (was: Signal type)
Organization: University of Illinois at Urbana-Champaign
Date: Sun, 30 Aug 1992 04:07:24 GMT

waldin@lcs.mit.edu writes:

>I have seen the term "interlocking" used a lot lately, mostly in conjunction
>with signal descriptions.  I've figured out what block signals are but
>I'm not quite sure what an interlocking is.

An official definition of ``interlocking'' that appears in many railroad
operating rule books is as follows:

	An arrangement of signal appliances so interconnected that
	their movements must succeed each other in a pre-determined
	order.  It may be operated manually or automatically.

The purpose of an interlocking is to prevent a signal from being cleared 
that would allow an unsafe move to result.  The interlocking apparatus
ensures that, in order to establish a train movement, all switches (turnouts)
in the route to be cleared be locked and that no conflicting routes are
cleared.  Before allowing a turnout to be moved, the apparatus ensures that
no train is occupying the turnout or has a favorable signal to proceed over
it.  In practice, there's a lot more checking involved in interlocking, but
I'm omitting the details for simplicity.

In the early days of railroading, train moves at junctions and crossings
were often governed by hand or light signals from a person posted at the
site.  Unfortunately, nothing prevented the switchtender from making
mistakes that caused trains to run into each other.

The interlocking was the result.  In order to give the signalman a good
vantage point of approaching trains, interlockings were typically placed
in towers.  Each appliance (signal, turnout, derail, etc.) was operated
by a lever in the tower connected to the appliance at trackside with pipe
or cable.  By means of interconnections between levers inside the tower,
moving a particular lever would mechanically lock other levers in position.
This protection prevented the signalman from moving levers that connected to
appliances that were not then safe to move.  The term ``interlocking'' is
derived from the mechanical interference between levers controlling
conflicting functions.

Beginning early this century, the mechanical locking was supplemented with
electric track circuits, electric switch machines, etc.  In later years
(the 1940's), it became the practice to perform ALL the safety functions
with electric circuits using relays; these later implementations are
more compact and easier to operate because they eliminate the need
for the mechanical apparatus traditionally used.

There are still examples of mechanical interlocking in use today but
they are gradually being replaced.

Centralized traffic control (CTC) is technically a form of interlocking
because of the inherent fail-safe checking done as part of CTC.  In CTC
lingo, a `control point' refers to a particular location operated as a
unit; each control point is really a miniature interlocking.  The
distinction between interlocking and CTC is often made because of the
application of different operating rules in each case; however, the
protection provided in both is nearly identical.  

>It appears to be a junction
>where lines cross or merge.  Is this right?  Is the place where a siding
>joins the main line also considered an interlocking?  Please help this
>confused individual.

Most interlockings exist at junctions or crossings.  However, junctions
or crossings that do not have the fail-safe protection I described are
not interlockings -- they are simply junctions or crossings.  The place
where a siding joins the main line is the most common form of interlocking
in CTC territory but in some places sidings can join a main line without
the benefit of CTC protection.
-- 
Jon Roma
Computing and Communications Services Office,
University of Illinois at Urbana-Champaign
Internet:  roma@uiuc.edu      	UUCP:      uunet!uiucuxc!uiuc.edu!roma