From: ajw@squid.mel.dit.CSIRO.AU (Andrew Waugh)
Subject: Re: Auto Gates
Organization: CSIRO, Division of Information Technology, Melbourne
Date: Sun, 3 Jan 93 23:36:45 GMT

In article <19010@mindlink.bc.ca> Steve_Hooper@mindlink.bc.ca (Steve Hooper) writes:
>> khartman@fnala.fnal.gov writes:
>> I have been interested in the methods used to "lift the gates" while
>> a train is close to a crossing, but not moving.  Setups that use the Diesel
>> horn to activate the gates, or a slow moving train/timer.
>
>I find this an interesting concept.  I have NEVER heard of such a thing....a
>gate that operates off horn sounds.  Not to call anyone a liar, but can you
>verify 100% that the whistle is what activates the gates?  I just find it
>strange that a noise would work the gates.  What would happen if a semi
>pulled up to the crossing and blew his horn?

While I certainly cannot verify that a particular level crossing in the U.S.
has horn activated gates, I can confirm that this method of operation has
been used in there.

The Railway Gazette of 4 February 1955 included a short article on such
an installation at Dumbarton on the Richmond, Fredericksburg & Potomac RR.
This level crossing had 4 tracks, some of which were used as head shunts
for a yard. While switching, the yard shunter would occupy the approach
track circuits for the level crossing. The shunter would rarely occupy the
crossing itself, and the gates would time out after 2 1/2 minutes of operation.
If, after the gates had risen, the shunter decided to move across the crossing
the gates could be reactivated by blowing the whistle. This was detected by
an ordinary carbon (telephone) microphone and a relay would pick up if the
sound was LOUD enough. This would restart the boom cycle.

The use of track circuits was considered, but coded track circuits were already
used with cab signalling and the addition of a number of short track circuits
would have been expensive.

>As I said, I could be 100% wrong here, but I can guarantee that in all the
>areas of BC and Canada that I have worked, I have never encountered a
>crossing acticvated by a whistle.

As far as I am aware, no Australian (or British) railway has ever used anything
similar either. In such a situation, i) the local railway would not allow
the booms to work regularly for 2 1/2 minutes if the train was not going
to enter the crossing -- this would be considered extremely dangerous; and
ii) a pushbutton would be provided adjacent to the crossing to start the boom
cycle when necessary for shunting.

>Perhaps (again, I say perhaps, because maybe I am completely wrong here) you
>are being fooled by a GCP.  A GCP is a Grade Crossing Predictor.  These are
>fancy gates that instead of having the usual activation block in the rails,
>actually detect MOVEMENT on the rails.  Thus, if you are going 10 miles per
>hour, the gates will go down when you are X amount of feet away from the
>crossing.  If you are doing 50, they will go down at Y feet from the
>crossing.  If you are stopped, they go up.  As soon as you start moving
>again, they go down.

A GCP is basically a TDR (Time Domain Reflectometer) a device known and loved
by those who maitain wiring plants such as ethernets. Periodically it sends
a pulse down the rails and listens for disturbances in the reflections
caused by a train approaching the crossing. (There are always some
disturbances; a fixed short is applied to the track at a certain distance from
the crossing so that the GCP can check its operation.) By timing the delay
before the reflection returns, the GCP can work out how far the train is
from the crossing. The next scan will give a new distance and by comparing
the two, the GCP can estimate when to start the booms/flashing lights
operating in order to give the optimum warning time (around 25 seconds).
(At least that is how the local variety work; there may be other types.)

It is a matter of considerable amusement to me that the first GCP in Victoria
(and probably in Australia) was installed on a tourist railway!

andrew waugh