Positive Train Control: A Technical Description

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.

How Engineers approach Risks

How Engineers approach Risk: A Discussion of Ethics

A building falls down. Now what?

In the United States, the first course of action is to figure out what went wrong. Was the collapse due to arson? A natural disaster? Or was the building designed poorly?

As a structural engineer, I will assume a lot of liability for any project I work on. It’s my responsibility to ensure that buildings stay safe during natural disasters. Every time someone enters a building I’ve designed, they entrust that I did my job right. Each building can be viewed as a promise that it will be a safe place to work, live, or play.

The scary thing is, safety cannot be guaranteed. What happens when a tornado strikes at the same time as an earthquake? It’s possible. And there’s no way to figure out the exact strength of a beam or column without breaking that beam or column in the process.

One answer would be to design every building so that they never fail, no matter what. But, as I mentioned before, there is a point where making columns bigger doesn’t actually make the building safer. The only thing accomplished by making buildings indestructible would be making them unaffordable.

So, torn between the two masters of safety and economy, engineers had to make tough choices. We had to decide how much risk the average person is willing to take, in every circumstance. For example, most buildings are designed to survive a fire only long enough to guarantee everyone in the building gets out. After that, the building has to be destroyed and a new one has to be built. Another example, we want our buildings to hold up during a 100 mph wind, but don’t expect them to survive 200 mph winds.

 

Engineers discussed these risks, how much they endangered buildings, and how much risk society was willing to take. These engineers also discussed statistical theory, and used that as much as possible to create their book of guidelines. The end result was the American Society of Civil Engineer's building code. The code is updated every year.

Inside the code, given forces, such as wind or earthquake, are multiplied by a safety factor. The riskier the force is, the larger the safety factor. The smaller the safety factor, the more willing we are to accept that risk. Inversely, the strengths of building materials are also multiplied by safety factors. These factors also vary, depending on how suddenly or drastically the beam or column fails.

Most of my academic courses have been focused on the countless ways for a building to fail, and how to design with each type of failure in mind. All of these methods have one thing in common: use the building code. It is my ethical obligation as an engineer to insure that the projects I work on obey this legal code. It is also my responsibility to look beyond the code to ensure my buildings won’t fail.

This is what it means to be an ethical engineer.