How It Works: Railroad Signals on the Northeast Corridor
The recent derailment in Philadelphia has brought to the forefront a safety critical system that usually functions in the background: railroad signals. Although the main purpose of signal systems is to prevent train collisions, they’re also part of the foundation of efficient and safe operations. Railroad tracks are divided into segments known as blocks. Without signal systems, far fewer trains could move through each block.
While originally intended just to inform the engineer that the track ahead was intact and unoccupied, automatic signaling systems have evolved to provide additional information, to limit train speeds, and to automatically stop trains when required for safety.
Modern signal systems can be thought of as a pyramid, the first level of which is the trackside signals of Automatic Block Signal System. These colored lights, singly or in combination, signal the engineer to take needed actions but do not override him or her if no action is taken. The next level of the pyramid is Cab Signals, which duplicate the indications of the trackside signals. Cab Signals are helpful in bad weather, when trackside signals cannot be seen or when conditions change after a train has passed a wayside signal. Like trackside signals, Cab Signals are not designed to take control of the train if the engineer fails to do so.
The third level of the pyramid is Automatic Train Control (ATC), which automatically slows or stops a train if the engineer fails to comply with speed reductions required by the cab signal. While this system can enforce required reductions in speed to 20, 30 or 45 mph to comply with signal indications, it is not capable of enforcing a speed restriction on a curve or stopping a train at a Stop Signal if the Engineer fails to do so.
The top of the pyramid is Positive Train Control (PTC). Amtrak’s PTC system is known as the Advanced Civil Speed Enforcement System (ACSES). It is currently installed and operating on 206 of the 401 miles of track that Amtrak is responsible for on the Northeast Corridor spine.
ACSES has several components that build on the protection provided by ATC. Among other things, it can automatically bring a train to a stop at a red signal or slow it on a sharp curve. Amtrak has supported the federal requirement to make PTC mandatory on certain passenger and freight tracks by the end of this year and is on schedule to meet that deadline. No other Class I railroad in North America is as far along in installing PTC systems as Amtrak.
Amtrak has had ATC in service for its trains on the Northeast Corridor (NEC) since it took over operations in 1976. In 1988, the Rail Safety Improvement Act required freight and commuter trains operating on the NEC to be equipped with ATC.
In 1991, Amtrak voluntarily took steps to modify its ATC system to enforce civil speed restrictions on curves. With support of the Federal Railroad Administration (FRA), Amtrak installed changes at locations where the maximum authorized speed approaching the curve exceeds the overturning speed on the curve. This step was taken as a result of a consensus among railroad safety leaders and stakeholders. Commuter railroads operating in Boston and the New York area made similar changes.
Last week, FRA directed Amtrak to take the following steps:
• Modify the ATC system at the location of the Train 188 derailment to automatically limit train speed on the curve on which the accident took place. This was accomplished before the first train moved over the curve on Monday, May 18.
• Assess the risk at all curves on NEC: Analyze all curves on the NEC to assess risk. In areas where approach speed is significantly higher than curve speed, the appropriate technology intended to prevent over-speed derailments must be implemented immediately. Amtrak must also take a new look at all curves along the corridor and determine if more can be done to improve safety in any of these areas and report back to the FRA with its findings. The effort began promptly after notification from FRA and is on-going.
• Increase wayside signage alerting engineers of the maximum authorized speed throughout the NEC. This effort is also on-going.
Amtrak continues to coordinate with FRA and is committed to the highest levels of safety for its customers and employees.