There are a number of different technologies applied to the
activation of grade crossing protection.
The simplest ones just put the gates down when ever there is an
occupancy in a certain section of track approaching the crossing.
There are newer and fancier devices that will give exactly 36
seconds from activation to train arrival. IIRC that is the FRA
standard.
The gates go up because the approaching train has cleared the
crossing.
The gates go up when a train stops short of the crossing because
the circuitry has a timing function that can detect that there has
been no movement.
At some locations there is a feature that allows someone with a
switch key to put it into a slot in a metal box and activate the
crossing. For quite obvious reasons I have never heard of any
location where the same function will deactivate the crossing
protection.
Think of the track at the crossing as divided into three sections:
An approach from the North.
An "island" right at the crossing.
An approach from the South.
In the simple systems the crossing is activated as soon as a train
occupies either one of the approaches. When it clears the island
the activation is ended.
On the motion sensing systems the hardware and software is looking
in both directions all the time. When a train enters either approach
it completes an electrical loop. As it gets closer the loop gets shorter.
The motion sensor can calculate the rate of change and adjust to that
36 second number at any train speed. If the train stops there is no
longer any rate of change and after a little while it will turn off the
activation. When the train starts moving again the rate of change will
start again and the crossing is activated. Motion sensors generally do
not use islands. Don't need to. If the electronics can detect an
approaching train and calculate it's speed then they can also detect a
train moving away from the crossing by a reciprocal rate of change.
If there are no insulated joints at both sides of a crossing it usually
means it has a motion sensor and does not need an island.
If the trackage involved is signaled the signal system and the crossing
protection coexist by using different frequencies and filters.
----- Original Message -----
From: "wollffee" <wolfee@onebox.com>
To: <BRHSlist@yahoogroups.com>
Sent: Saturday, 11 October, 2003 08:39
Subject: [BRHSlist] How does a railroad crossing work?
> What triggers the lights/bell/crossing gate when a train approaches
> the crossing? How come when the BNSF approaches the crossing at 80th
> and Sheridan, NW of Denver, it triggers the crossing a quarter mile
> before it gets there--even a coal train chugging at 5 mph. But, once
> it passes, the gate goes up almost instantly after the train is
> thru? Sometimes a southbound pulls right up to the crossing and then
> stops. After he gets stopped, somebody does something to let the
> gate back up while the train just sits there. How do they make the
> gate go back up when the train is parked just a few feet from the
> crossing?
>
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
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