From: roma@ux1.cso.uiuc.edu (Jon Roma)
Newsgroups: rec.railroad
Subject: Re: Note on Newport News Wreck
Date: 19 Aug 92 22:40:58 GMT
Article-I.D.: ux1.Bt95oo.JAI
Organization: University of Illinois at Urbana-Champaign

ehr@ecsvax.uncecs.edu (Ernest H. Robl) writes:

>In article <1992Aug19.084731.8424@mailhost.ocs.mq.edu.au>, eoliver@laurel.ocs.mq.edu.au (Eddie Oliver) writes:

>> Some recent postings have suggested that it is possible in US/Canadian power
>> signalling areas for a switch to be moved after the train has passed the
>> protecting signal.

>> In every safeworking system I know, the releasing arrangements for switches
>> in power-signalling areas are arranged so that you cannot move the switches
>> indiscriminately in such a manner. The occupancy of the relevant track 
>> circuits will prevent the switches from being released, typically until
>> some conditions are specifically tested to ensure safety (e.g. until a
>> short approach track has been occupied for some sufficient time period).

>> What am I missing? Is such a precaution not provided in US/Canadian
>> installations?

>There is a difference between switches which are part of an 
>interlocking and fully electrically locked and switches which are
>not part of the routes controlled by the CTC installation.

>In the first case, a variety of logic tests govern whether the
>switch can be thrown and the switch position is in turn reflected
>by signal indications.

>In the second case, the switch is only locked mechanically, with a
>switch lock.  It is, however, connected to the signal system so 
>that if it is not properly lined for the main route, the signal
>governing entrance to the block will show a restrictive aspect.

>In this second case, the siding is not used for meets but for 
>setting out cars, such as for a team track and industry loading
>area.  In normal operations, the train is already in the block
>when it stops for switching operations and the switch is thrown
>to provide access to the siding.

>The electrical connection for monitoring the switch position is
>normally only for safety purposes -- not for route control.  In
>most cases, trains are not allowed to clear (completely leave the
>mainline tracks) at these points.

All of what Ernest wrote is correct.  However, most industry switches and
non-interlocked crossovers in CTC territory are equipped with electric
switch locks which prevent the switch from being opened if the adjacent
block is occupied.  Unlocking the door on the lock box throws the adjacent
signals to Stop and initiates a time delay intended to ensure that any
trains in the vicinity have had an opportunity to come to a stop.  At the
expiration of the time delay, the energization of an electromagnet releases
the lock.  Subsequently, the turnout may be operated freely by hand until
the electric lock box has been closed and re-locked.

In the event the block governed by the electric lock is occupied, the lock
will not release unless the train is within a short clearing circuit running
a few car lengths from the switch.  This allows a train standing by the switch
to open it to perform switching without the time delay.  This clearing
circuit is often of the audio frequency overlay (AFO) type superimposed on
the regular track circuit.

FRA requirements put a 30 mph speed limit on sections of CTC territory
that have switches that are not electrically locked.  From the speed of the
train involved in the recent Virginia wreck, I would presume that the switch
in question WAS electrically locked.

In order to sabotage such a switch -- assuming this is what happened -- the
perpetrators would have had to falsely energize the locking device in order
to allow the switch to be opened in the path of an oncoming train and/or
would have had to circumvent the switch point detection so the switch
position wouldn't affect the signal indication.
-- 
Jon Roma
Computing and Communications Services Office,
University of Illinois at Urbana-Champaign
Internet:  roma@uiuc.edu      	UUCP:      uunet!uiucuxc!uiuc.edu!roma