Positive Reinforcement

May 14, 2015

Boston takes the lead with train controls on its Green Line transit

Although light-rail transit systems are not included in a 2008 federal law mandating positive train control (PTC) adoption, the Massachusetts Bay Transportation Authority (MBTA) plans to retrofit its Green Line with the safety system, providing a model for systems nationwide.

At 200,000 passengers each day, Boston’s Green Line is the highest-capacity light-rail system in the country. Its owner, the MBTA, also wants to make it one of the safest.

At 200,000 passengers each day, Boston’s Green Line is the highest-capacity light-rail system in the country. MBTA wants to make it one of the safest.

Safety is the top priority for the MBTA’s board of directors, as well as former general manager and now secretary and chief executive officer for the Massachusetts Department of Transportation, Richard Davey. The Green Line has been the site of two high-profile accidents in recent years. In May 2009, a trolley car driver, who was texting, rear-ended another trolley. The incident sent 49 people to the hospital. One year earlier, in May 2008, an operator, apparently suffering from a microsleep incident, drove her train past a red signal and into the rear end of a stopped train.

The 2008 incident prompted a letter from the National Transportation Safety Board (NTSB) advising the MBTA and other transit agencies to consider PTC even though light-rail transit is not included under a 2008 federal law mandating PTC adoption. The NTSB had long been calling for safer systems, even including PTC on its “most-wanted” list of transportation safety improvements. The MBTA is one of the first U.S. transit agencies to respond positively to the NTSB’s request by considering retrofitting its Green Line with the safety system.

“Although PTC is not mandatory for light-rail transit systems, I believe light rail eventually will follow suit because of the increased safety PTC provides,” said Mike McInnis, HNTB project manager. “When it does, the Green Line will be a model for success.”

Transit intervention

The Green Line has tunnel, aerial, at-grade and mixed-traffic sections. In-street sections operate by line-of-sight, which means a train operator has total control of the vehicle and drives according to traffic signals and conditions that he or she can see ahead. In signaled sections, the vehicle still is under driver control and can be driven through a red signal without system intervention.

With PTC technology, if an operator does not slow a vehicle that is in danger of collision, the vehicle slows itself. PTC’s predictive technology monitors a train’s current conditions against its speed and end-of-authority limits. The speed and authority limits are monitored by the train’s speed curve and a distance-to-target (counter down) or the train’s breaking curve of the onboard PTC segment. PTC is activated when the train is no longer operating at an allowed speed (track speed or civil limit) or is not slowing or braking sufficiently to stop short of the end of authority.

Once in place, the Green Line PTC system will serve as a template for light-rail transit systems across the country due to the following factors:

  • Continuous monitoring of train locations with positioning technology;
  • The ability to automatically override dangerous train movements; and
  • Stopping a train if the crew cannot.

In its move toward implementing PTC on the Green Line, the MBTA hired HNTB Corp. to provide design engineering services, including conducting an alternative analysis and recommendations study. As part of the study, HNTB will interview suppliers and collect best practices by visiting five properties with operational PTC systems. The study’s purpose is fourfold:

  • Define PTC as it relates to light-rail transit and the Green Line;
  • Identify candidate PTC systems that meet the Green Line’s unique operational characteristics;
  • Determine how PTC will affect operations; and
  • Outline how the system will be implemented.

HNTB is an active member of the Railroad Safety Advisory Committee established by the Federal Railroad Administration to develop new regulatory standards, including PTC. As part of that committee, HNTB has fundamental knowledge of PTC. HNTB will develop the scope for preliminary engineering based on the study’s recommendations, which are due in March 2012. The type of system the MBTA chooses and the system’s cost, which can range from $12 million to $20 million per mile, will determine the operational date.

The Green Line has tunnel, aerial, at-grade and mixed-traffic sections. With PTC, if an operator does not slow a vehicle that is in danger of collision, the vehicle slows itself.

“We are plowing new ground in PTC implementation, and we want to make sure we do it right,” said Michael Turcotte, MBTA assistant general manager, engineering and maintenance. “Our goal is to make the Green Line the safest light-rail line in the country.”

Hang together

Being one of the first U.S. light-rail transit systems to voluntarily consider PTC implementation carries a significant risk.

“There are scores of academia, vendors, major railroads and commuter properties trying to answer the same questions we are,” McInnis said. “One of the biggest risks is that the MBTA could go through the entire exercise and come up with a different solution than they do. Being excluded from the federal PTC mandate does not exempt the Green Line from creating a PTC system consistent with the rest of the industry.”

Some attempts at similar advanced technology have resulted in decade-long pilot programs. McInnis hopes to achieve a cohesive solution and avoid protracted pilot programs by recommending that the MBTA work collaboratively with other transit organizations toward a unified solution.

“What the MBTA does ultimately will become a transit-industry best practice, so it makes sense to ask the American Public Transportation Association and others to share in the slings and arrows of development,” he said.

McInnis’ suggestion is not unlike what the major freight railroads have done for PTC development.

“All of the railroads contribute infrastructure components to this big test bed in the middle of the desert,” he said. “They run trains around it and see what the impacts are. Everyone contributes, and everyone shares in the benefits derived from it.”

Travel-time drag

Implementing PTC means the MBTA may face efficiency issues. PTC is a speed-control system. It will make the Green Line safer but possibly slower, which means passengers may no longer view it as efficient travel.

To offset the slower speeds, HNTB hopes to identify ways to increase the system’s capacity. If trains carry more people, or if the system could operate more efficiently, the MBTA might be able to compensate for PTC’s drag on travel times.

“To address the problem, we have to change our perspective,” McInnis said. “What’s important here is not moving trains, it’s moving people.”

HNTB is enlisting a two-part strategy to solve the safety vs. speed issue. First, McInnis and his team conducted a “Day in the Life of the Green Line” survey of train operations and how they affect ridership. The survey’s mission was to identify ways to make operations more efficient. For example, one segment examines train dwell times at station platforms, documenting how trains behave when they pull into stations.

MBTA hopes to avoid protracted pilot programs by recommending that the agency work collaboratively with other transit organizations toward a unified solution.

“Does the operator pull up to the end of the platform? Does another train pull up right behind that one? What doors do they open? And, how long are they open?” McInnis said. “We are analyzing the data to find out what is causing constraints, and then we will make recommendations to improve capacity.”

The “Day in the Life” survey data will be loaded into a database that incorporates information from the Automatic Vehicle Identification system, the Automatic Fare Collection system and the Central Transportation Planning Staff, an arm of the Massachusetts Department of Transportation that conducts ridership surveys.

“If you combine all of the sources, this survey is probably a first of its kind,” McInnis said.

Part two of the survey involves loading the database of findings into train-operation simulation software to determine how many people the system is effectively moving. The simulation will test capacity-increasing options, such as expanding train size and dynamic double berthing, where two trains alight and board passengers simultaneously on the same track.

The OnTrack simulation software’s developer is now working to make enhancements, so the software will simulate PTC and factor in ridership, as well as trains.

MBTA conducted a “Day in the Life of the Green Line” survey of train operations. The survey’s mission was to identify ways to make operations more efficient.

“HNTB’s OnTrack network simulation capabilities and Green Line’s system database will help the MBTA achieve the optimal combination of safety and operational throughput,” Turcotte said.

The MBTA will evaluate the success of its Green Line PTC system on four fronts:

  • Decreased accidents;
  • Implementation costs;
  • Effect on operations; and
  • Overall rider experience.

“The biggest unknowns are cost and financing, and the impact PTC will have on train operations,” McInnis said. “The goal is that our study and recommendations will provide the industry with a mix of solutions to address all of these things.” TM&E

About The Author: McInnis is the associate vice president and manager of railway engineering for HNTB Corp.

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