Positive Train Control: A
Technical Description
In October 2008, President George Bush signed into
law the Rail Safety Improvements Act of 2008. Chief among its requirements was
forcing all Class I railroads to use Positive Train Control, or PTC, on their
railroads by 2015. While there exists many documents and opinion pieces that
speculate on how this will affect the future of railroading, very few of these
explain what PTC is. I hope to fill this gap by exploring two main PTC systems
that are currently being implemented in the United States.
After I first heard of Positive Train Control, I
was confused by the lack of explanation on how it works. These explanations
don’t exist because PTC is not a specific technology. Rather, it is any
technological system that accomplishes four specific goals.
First, PTC must keep trains separated by a
reasonable distance and prevent collisions. Trains are heavy. As a general rule
of thumb, it takes a train one mile of distance to come to a complete stop.
Most sections of track do not offer this visibility, which means accident
prevention techniques must be proactive, as there is no reactive course
available. Keeping trains separate is a clear case of proactive action that
railroads should have complete control over. PTC will be another tool to help
this happen.
Second, PTC will enforce line speeds. Railroads
are just like roads, with speed limits. On top of these limits are reduced
speed zones around corners or in school zones. The same applies to trains, and
when trains speed around a curve or over a switch, derailments are likely.
Third, PTC must enforce temporary speed
restrictions. Again like roads, railroads impose temporary speed restrictions
in unsafe environments, like areas with track construction, or in bad weather,
or when there might be boulders on the track or other natural hazards.
Fourth, PTC must improve rail worker lineside
safety. Construction workers and other employees frequently work next to active
mainlines, and accidents happen. PTC must offer a method to decrease these
accidents.
Historically, to accomplish these goals, railroads
have used semaphores, which are the railroading equivalent to traffic lights.
Semaphores typically have three settings. The first is clear, where the line
ahead is open and the full speed limit is in effect. The second setting is
caution, where the train is allowed to enter but at a reduced speed. Some
railroads have multiple caution settings to better address the variety of track
conditions. The final setting is stop.
The goal of Positive Train Control systems is to
display semaphore signals inside the locomotive cab, and force the locomotive
to slow down or stop when it’s going faster than the posted speed. To
accomplish this, there needs to be a computer on the locomotive that compares
how fast it should be going to how fast the train is moving, and a method of
telling the computer what speed is safe for its current location.
In order to meet these requirements, railroad
companies are looking at two unique technological approaches. The first
approach is to install radios on semaphores to talk with the locomotive
computer, relaying track conditions and the locomotive’s location. The second
method uses the railroad track themselves as giant wires to relay the
information.
Using railroad tracks to electrically relay
information is not a new idea. Most crossing guards (the flashing lights
installed where track and road meet) are triggered when an approaching train
shorts the gap between the two rails. More to the point, such a system has been
in use on what is now Amtrak’s Northeast Corridor ever sense the 1920s.
In 1922, the Pennsylvania railroad launched an
ambitious project to install in the cabs of their locomotives a system to
display semaphore signals inside the cabs of their locomotives. The system
worked by sending electromagnetic pulses along the rails, and a sensor aboard
the locomotive could read the frequency of the pulses. A rate of 180 pulses per
minute (ppm) meant the track was clear, 120 was Approach Medium, 75 ppm for
Approach, and 0 ppm for Stop. In this way, the system was failsafe, as a
breakdown would bring all traffic to a stop. This system was implemented on
their mainline between New York City, Philadelphia, Baltimore and Washington
D.C. It was a success, and became the de facto standard system in the world.
In 1992, Amtrak updated the system to display
higher allowable speeds to newer locomotives. The higher speeds used a 250
hertz bandwidth instead of the original 100 hertz, so older equipment wouldn’t
accidently read the new signals. Because the old signals were still transmitted
in addition to the new ones, the system is backwards compatible without having
to update old equipment.
Other lines are looking at a fresh approach to implementing PTC. Instead of using the track, their goal is to use wireless communication, either radio or cellphone frequencies, to broadcast communications directly to the train. Radios offer the benefit of
being able to transmit information in two directions, across great distances
and without using much power. Radio also allows for messages to be encoded,
letting locomotives ignore signals that don’t match their identity. One
broadcaster could communicate with multiple locomotives.
The challenges lie in finding a single frequency
that can be applied to every railroad. Modern railroad practices share
locomotives. Any PTC system has to be accepted by all eight major railroads in
North America, and then installed across every mile of track in the United
States.
In spite of these challenges, the general
consensus is that Positive Train Control will be installed by the 2018
deadline. Once it is installed, it opens the possibilities for an exciting
future. PTC can replace the current block system, allowing trains to pass
closer together and use the track more effectively. Onboard screens can display
upcoming speed changes, allowing for a more complicated series of speed zones
because the engineer no longer has to memorize them. This can maximize the safe
allowable speeds on a route and increase the average speed of the train. By one
study, these changes could increase the number of loads a railroad can haul by
20%.
Positive Train Control has two basic functions.
First, it is an information system designed to keep engineers informed on safe
allowable track speeds. Second, it is a failsafe designed to slow the train
down should it exceed the allowed speed. PTC isn’t a specific set of
technology. In fact, there are multiple systems that achieve this end. Finally,
PTC, if explored, can open up opportunities for those in the business of
railroads.
Sources:
Association of American Railroads. “About PTC.” https://www.aar.org/policy/positive-train-control
Johnston, Bob. “Amtrak 188: One Year Later” Trains Magazine May 2016: 6-7. Print.
Frailey, Fred W. “Positive Train Control: Oh, what a mess” Trains Magazine March 2014: 16-17.
Print.
Hansen, Peter A. “6 High-Tech Advances” Trains Magazine November 2008: 26-29. Print.
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