Paul,
That is a very well written explanation of how brakes work. I find it
interesting that you guys do a brake pipe leakage test. Do you guys not use an
air flow meter on your units to monitor brake pipe leakage? Just curious.
On CPR, all units are equipped with air flow meters.
Paul Smith
Winnipeg
To: cpsig@xxxxxxxxxxxxxxx
From: cpsig@xxxxxxxxxxxxxxx
Date: Tue, 27 May 2014 20:08:39 -0500
Subject: [cpsig] Re: Fw: Central Maine & Quebec CEO: 'We'll have to prove
ourselves'
Been watching this topic for some time and find some of the conclusions very
interesting. I thought I'd start my comments by explaining exactly what goes on
with Railroad airbrakes from the time the Locomotive couples on till it cuts
off. This will be a long message and I'll probably get into trouble with
somebody because I will no doubt leave something out, but here goes.
When air is applied to a train from either the locomotive or yard air line the
control (triple) valve on the cars sense the increasing pressure and (a)
release any brakes that are still applied and (b) charge the service and
emergency portions of the reservoirs. The train line of course is also charged
to whatever the prescribed pressure is on that particular Railroad. Usually
that's 80 or 90 pounds per square inch for freight trains. Passenger trains may
be as much as 110 psi.
When the train line and locomotive equipment reaches full charge an airbrake
test is made. These days that means that the engineer will make a 20 pound
service reduction with the brake valve and when the air stops blowing he cuts
out the feed valve and watches the equalizing reservoir for one minute. If
leakage is not more than 5 psi then he can release the brakes, cut in the feed
valve and the air test is complete. It's also necessary to note that the brakes
apply and release on the rear end. Sometimes this is with a carman or crew
member and it can also be done with an ETD device.
The way the system works is that each car has a two part air reservoir (service
and emergency) which supplies air pressure to the brake cylinders when the
train line air pressure is reduced. Generally for regular service freight brake
applications you get about 2.5 pounds pressure in the cylinder for each 1 pound
reduction in the train line. If the train line is vented in any way at more
than a specified rate than the triple valve will also kick in the emergency
portion of the air reservoir into the cylinders.
Freight train airbrakes once applied can only be increased or totally released.
Passenger train brakes have a graduated release function which allows the
engineer to decrease the breaking effort as the train slows down. This
graduated release function is why Passenger trains can be brought to a slow,
easy stop without a jerk at the end.
Some airbrake equipment releases faster because once the engineer releases the
airbrakes air pressure from the cylinder ( not the reservoir) is put back into
the train line. But that's only if the release is initiated from the head end.
There is no circumstance I know of where air pressure from either portion of
the car reservoir is put back into the train line. Air pressure flows into the
reservoirs from the train line and only exits from the reservoir to the
cylinders unless the car is bled off manually.
If I correctly understand what happened in this incident the engine was left
attached to the train after the brakes were released. In that situation with
the brakes applied one would have to place the brake valve in the release
position for the locomotives to release the brakes. Assuming the locomotive
brake equipment is functioning normally neither the pressure maintaining
feature nor any handling of the locomotive independent brake valve would cause
the train brakes to release.
Same for the ETD equipment which use air pressure from the train line to run
the small electrical generator. That equipment uses a small amount of air from
the train line. It does not bleed air from the reservoirs and therefore cannot
cause the train brakes to release. At most there would be a small increase in
the brake cylinder pressure because of the slight reduction in the train line
pressure.
Normally, trains or for that matter cars set out would leave the brake valve in
emergency position and or leave the train line angle cock open so that there
can be no increase in train line pressure to release the brakes.
There was some speculation early on that the train ran away after locomotives
died or were shut down. Because of the way the airbrakes work shutting down the
locomotive would not release the airbrakes once they were set.
The issue that I see here is that a train or cars were left where they could
get to the mainline on a steep grade. That is to me the relevant issue. Cars
and/or Trains are set out and left along mainlines and sidings all over north
America. But, leaving cars unattended on steep grades is another issue all
together. When cars or trains are set out a sufficient number of handbrakes
must be set to prevent movement. I know everybody wants to hear a specific
number but that is impossible to say in advance. It all depends on the train,
train weight, location, grade, even wind and other conditions that cannot be
determined in advance.
Also, one cannot necessarily depend on the locomotive brakes to hold the train
still. For example, for several years I worked as trainmaster/asst supt between
Barstow and Fullerton California. That territory includes Cajon Pass which has
a westbound 2.2% descending grade from MP 54.8 to MP 81. Locomotives on that
territory can hold approximately 1000 tons per locomotive unit still without
the train brakes or handbrakes being applied. I.E. If you have 4 locomotive
units and 5000 tons when the train brakes are released the train will start to
move even though the locomotive brakes are fully applied. One track on this
territory also includes six miles of 3% descending grade. The employee
timetable for that territory includes a number of rules about how cars may be
set out, what handling must be given, setting handbrakesand/retainers etc, even
when the train has a crew on it. In some unusual cases on steep grades it may
even be necessary to set both airbrakes and retainers to control the speed of
the train until the engineer has control of the brakes and the train is
recharged. Needless to say trains are not left on this territory unattended.
Cars that have to be set out have the handbrake securely tied and sometimes
skates are applied under the wheels to keep the cars from moving. Setout tracks
usually have a switch at the lower end which can be lined away from the
mainline so the cars do not roll far enough to foul the main track.
The act of cutting the locomotives off from the train if not properly done can
result in a pressure wave which will release the train brakes after the
Locomotive is gone but that didn't happen in this case as locomotives were
never cut off.
I hope the above helps clarify what Railroad airbrakes are and how they work.
Much of the above is required by the rules and in the US at least is federal
law.
Sent from my iPad
Paul Nash
www.pbase.com/paulv99k
