Saturday, June 13, 2009

High Line Park: A Most Unusual Rail-to-Trail Project

The High Line Park opened this week on the west side of Manhattan. This is probably the most unusual and unique rail-to-trial conversion in the US.

Why? Perhaps because it’s located 30 feet above the city streets. Perhaps because it’s a rail line that few knew it was there. Perhaps because it may be the only rail-to-trail linear park entirely on an elevated steel viaduct.

Also, it is a new approach to the creation of a linear park. The linear park, pioneered by Fredrick Law Olmsted with Boston’s Emerald Necklace, is typically characterized by one or modes of transportation facilities weaving through a long park corridor. High Line Park weaves a pedestrian esplanade with park landscaping within the linear corridor of this Westside viaduct.

High Line Park was one the New York Central Railroad’s West Side fright line, high above 10th Avenue in Manhattan. It was built in the 1930s to replace the street-running tracks that served the Hudson River piers and other industrial customers. The railroad used men on horseback, nicknamed “cowboys,” to guide the trains through the city streets.

The elevated line brought two improvements:
1. It removed the conflicts of running freight trains through busy city streets
2. It was electrified and eliminated the use of steam engines

The elevated line served many west-side customers for years. But with the dawn of interstate highways and the shifting of freight from trains to trucks, the line feel into disuse in the 1970s and abandonment in the 1980s. Interestingly, the out-of-service line remained in the ownership of Conrail and was even transferred to CSX in the Conrail breakup.

As with most unneeded and abandoned structures, the High Line was scheduled for demolition. However, others had another vision. What some saw as an eyesore, others saw as a most unique and interesting “place.” It is above and away from the bustle of the sidewalks and streets. It provides a unique view of the city. And it is an opportunity to provide new open space without demolishing any buildings to create it.

The Friends of the High Line and others advocated for the vision of a park for years. Finally, it has become a reality, as the first segment opens.

For more information about the park, the history of the High Line, and great photographs and videos, visit the Friends of the High Line at the website: .

Wednesday, June 3, 2009

How to Safely Control Trains on the Green Line

Following the recent MBTA Green Line crash at Government Center, and with another similar crash in recent history on the Riverside Line, a number of people have asked “Can it have been prevented?” In other words, could a more modern type of signals and train control have made it impossible for one train to crash into another?

Right now there are signals in the subway and out on the Riverside Line. But, unlike other lines, there is no “enforcement” of the signals. On the other subway lines and on major rail lines (like Amtrak’s Northeaster Corridor), a train that passes a red light will automatically have its breaks come on.

There is no such system for the Green Line. But would a signal system with “enforcement” work on the Green Line?

The short answer is that such a train control systems could be used, but I don’t think it would work on the Green Line. Such a system would keep the trains further apart. With trains running 90 seconds apart, they would back up and the Green Line wouldn’t handle the approximately 240,000 people a day that use it.

Train Control Systems - A Brief Overview
What most people know about tranit signals are the red-yellow-green lights in the subway. The indications are similar to traffic signals for motor vehicles.

In addition to the signals themselves, train control often has a component of “enforcement.” The simplest feature is the trip stop – a little arm next to the track that is up when the signal is red. The arm sets off the train brake, preventing it from going through the red signal.

In more complex systems, the signal system can interact with the train controls. Sometimes called “civil enforcement” a train control system will not only stop the train before it goes through a red signal, but also will slow it down if it’s going over the speed limit. [Can you imagine that in automobiles? You’re on the MassPike going the limit of 65. You want pass a car, and speed up to 70, but the “enforcement” comes on and slows you back to 65!]

This “civil enforcement” is used on most of the Red and Orange Lines and on high-speed lines, like Amtrak’s Northeast Corridor.

The Green Line has on “civil enforcement” and that is part of the question: should it be used on the Green Line.

Train Control on the Green Line Today
In general, the MBTA Green Line employs 3 types of train control:

1. Automatic Wayside Block Signals: These are the red-yellow-green signals along the side of the tracks.

2. Line of Sight: This is used for street reservations (like the median of Commonwealth Avenue, Beacon Street, or Huntington Avenue) and for street running (e.g., Huntington Avenue and South Huntington Avenue outbound of Brigham Circle.

3. Traffic Signals: Where there are traffic signals at intersections where the Green Line runs in a street reservation or in the street, these traffic signals also govern the operation of the trains.

Automatic Wayside Block Signals
The red-yellow-green automatic wayside block signals are used on the Lechmere Viaduct, Central Subway and on the Highland Branch (Riverside Line, D Branch). This system employs a three-aspect (red, yellow and green), two-block system. Wayside signals will display red if the track circuit immediately beyond the signal is occupied. Signals will display yellow if the next signal in the direction of operation displays red and the intervening track circuit is unoccupied. Signals will display green if the next signal in the direction of operation displays yellow or green and the intervening track circuit is unoccupied (i.e. a minimum of two signal blocks ahead are clear).

Line of Sight and Traffic Signals
When the Green Line runs in the street (like South Huntington Ave.) or in a street reservation (like Commonwealth Avenue), there is no signal system. Rather, operations are be governed by what is called “operator line of sight.” This is no different from anyone driving a car.

Just like driving, there are a series of rules that the operator must follow. And, like driving a car, safety is highly dependent on the operator’s judgments and actions. The operator must obey the traffic signals and also maintain a safe separation from the train in front of them.

What Works and Doesn't Work for the Green Line?

The Green Line is the most heavily used light rail system in the nation. With over 240,000 daily riders, it's four branches converge in the Central Subway, where trains run an average of 90 seconds apart at rush hour. Any regular rider of the Green Line knows how close the trains operate. Often as one train leaves a station, the next one is entering.

Also, the trains tend to bunch up, something know as "platooning." When my firm was working on the light rail accessibility study, we modeled the Central Subway. Even when we started the model with every train running right on time, the model showed that give it an hour or so and the trains will bunched up!

This bunching means that trains may be running at about 45-second headways while bunched, and a few minutes between the bunches. In other words, there is a statistical distribution and the 90-second headway is only the statistical average.

Why is this important? Because, a signal system with train control would not support headways as little as 45 seconds in the Central Subway. Such as system looks at the breaking distance, the distance it takes a train to stop. The system ensures that the train will stop before it can possibly hit the train in front of it. This would keep the Green Line trains further apart. It would reduce the number of trains that can pass through the subway in an hour.

On the other hand, such as system might work on the Riverside Line where speeds are higher and trains run about 5 min. apart at rush hour. Likewise it might be applicable to the new extension to Medford. But the MBTA is studying this, and we should await the study results to see if it would work.


In conclusion, I agree that safety may need to be improved in the wake of recent accidents. However, a train control system does not appear feasible for the subway, due to the tight headways that are needed to get all the trains through on time. It may be applicable for the Riverside Line and the new extension to Medford. But in any case, we should await the results of the ongoing MBTA study of how best to safely control trains on the Green Line.