THE Rail Safety Improvement Act of 2008 requires US railways to install Positive Train Control (PTC) on tracks that carry passengers or toxic-by-inhalation materials. The Association of American Railroads (AAR) estimates that more than 96,000km of track needs to be equipped, which involves the installation of PTC on thousands of locomotives, a new wireless communications network, back office systems, and tens of thousands of trackside devices connected to signals, switches, and other wayside equipment.

Transportation Technology Center Inc (TTCI), based in Pueblo, Colorado, has been working with the rail industry for 15 years to develop many of the technologies needed for PTC, including braking enforcement algorithms, data radios and networks, train location and integrity determination, broken rail detection, operations analysis and train control modelling, and testing.

TTCI has been involved with PTC since its inception, and some of TTCI's engineers played key roles in the development of the first PTC systems, including the Advanced Train Control System, the Positive Train Separation System, the "Eastern PTC" system, and the Incremental Train Control System.

TTCI's initial work on PTC research and development began in 1998 through hosting field tests of the Advanced Civil Speed Enforcement System (ACSES) on the PTC Test Bed at the Transportation Technology Center (TTC) in Pueblo.

In 2000, TTCI was appointed manager for the North American Joint PTC Programme aimed at developing an interoperable PTC system to enable high-speed passenger operation on the St Louis - Chicago corridor.

In 2007, TTCI began managing the Vital PTC project to address technical issues common to most PTC systems, such as improved methods and algorithms for enforcement braking, improved onboard location determination, and improved methods of testing. Both programmes yielded results which contributed towards today's PTC systems.

Because many early braking enforcement algorithms for freight trains were designed to be overly conservative to meet safety objectives, they often caused operational inefficiencies. TTCI has been researching how to resolve this since 2007 and has developed algorithm logic for a number of new techniques, along with a comprehensive braking algorithm evaluation methodology for testing.

The former includes an improved target offset function, adaptive functions that measure actual braking performance characteristics and adjust braking distance prediction based on this measured data, using the emergency brake as a backup to the penalty brake, an improved propagation time estimate for trains with distributed power, calculation of more accurate brake force in the back office, and incorporation of a dynamic brake currently used in predicting braking distance. Research into additional techniques continues, including adding support for wagons equipped with electronically-controlled pneumatic brakes, wagons equipped with empty/load devices, and specialised wagons.

TTCI has been developing and validating a high-fidelity passenger air brake performance model and will soon begin research and development to improve passenger braking enforcement algorithms.

TTCI has developed a simulation testing methodology for braking enforcement algorithms that allows any vendor's algorithm to be tested as a "black box" within a much larger range of operating scenarios than field testing alone could feasibly provide.

PTC-TTCIFor freight trains, a validated, industry-standard software application is used to perform a wide range of modelling exercises associated with train-level dynamics to conduct large-scale PTC brake enforcement tests for a broad range of operating scenarios. Each is simulated a few hundred thousand times, during which the parameters that affect the train's stopping distance are varied in a Monte Carlo method according to distributions representing real-world variability. This enables the evaluation of potential outcomes from a PTC penalty enforcement in each operating scenario tested, providing a statistical view of the safety and performance characteristics of the algorithm.

The PTC Test Bed allows field testing of braking enforcement algorithms within a secure and dedicated environment. It is also used to quantify the performance of braking enforcement algorithms for freight and passenger trains using actual hardware inputs and to validate the results of simulation tests.

For many years, TTCI has been involved in developing and testing wireless communication systems for PTC. During the Higher Performance Data Radio (HPDR) project launched in 2006, TTCI managed the development and testing of a data/voice radio using a railway's existing radio frequency spectrum while addressing the need for increased throughput to support voice and data communications, including those required for PTC.

The HPDR research project has accelerated development of the Interoperable Train Control (ITC) PTC 220MHz data radio used for PTC deployment. TTCI uses these radios in field testing of ITC-compliant PTC systems within its PTC Test Bed.

Today, TTCI is helping the railways design 220MHz radio frequency (RF) data networks for PTC. Because existing RF simulation modelling tools alone were unable to meet railway needs, TTCI developed a hybrid approach using a commercial RF propagation modelling package and tools developed by TTCI to simulate PTC message traffic and predict link loading. The results are used to develop intelligent and efficient RF design plans.

TTCI has been managing the research, development, and testing of a proof-of-concept Positive Train Location (PTL) system since 2010. PTL is an enabling technology for future applications, which include PTL providing future PTC onboard systems with a more dependable means of track discrimination and higher-accuracy head-of-train and end-of-train location. Table 1 illustrates PTL's relative positioning accuracy compared with GPS and GPS with differential correction. The PTC Test Bed and its Real Time Kinematic (RTK) GPS truth reference system with onsite reference station have supported PTL field testing.PTL-table

The High Tonnage Loop (HTL) at TTC's Facility for Accelerated Service Testing (Fast) hosts a test bed for fibre-optic-based detection systems.

Because the HTL experiences a rail break once every 1½ days on average, the HTL is ideal for testing broken rail detection systems. The test bed will evaluate the ability of fibre-optic systems to detect a broken rail before the rail fully separates. A track circuit-based rail break detection system serves as a "baseline" against which to compare the performance of fibre-optic detection systems.

The fibre-optic test bed is also used to explore the technology's capability to identify a train's location, length, and speed, to determine which track a train is on, and detect when a wagon may have errantly left a passing loop or siding.

TTCI uses simulation tools, such as Rail Traffic Controller (RTC), to perform operations analysis and train control modelling. RTC evaluates how changes to track configuration or operating procedures will impact train operations. These tools are also used to determine how various types of train control, mobile data communications systems, and braking enforcement algorithms may affect train operations.

The PTC Test Bed has been supporting PTC development and testing since 1998. Today, it includes PTC-equipped locomotives, back office equipment, signalling, wayside equipment, and communications systems required to represent revenue-service PTC operations, including ITC (also known as Interoperable Electronic Train Management System or I-ETMS developed by Wabtec) and ACSES II.

The ITC implementation features I-ETMS-equipped locomotives and ITC-compliant base stations, wayside interface units (WIU), and a back office server (BOS). Wayside locations are equipped with PTC 220MHz wayside radios and Wi-Fi and cellular communication paths and wayside messaging servers. To fully test ITC-compliant PTC systems, the PTC Test Bed includes intermediate signal locations, a passing loop with control points, and a computer-aided dispatch system. The test bed's two control points have a fibre optic link to the BOS and a local control panel to test the control points' response to messages sent by BOS.

The ACSES II system features ACSES-equipped locomotives, a nine-aspect cab signalling infrastructure with 12 blocks, and two interlockings with simulated WIUs. Passive transponders transmit track information, including a permanent speed restriction, and there are two base stations equipped with both 220MHz and 900MHz radios. In the back office, a temporary speed restriction (TSR) safety server emulator and its track database provide TSR data, while a field simulator provides interlocking status information.

The PTC Test Bed has an integrated simulation capability, which includes train simulators, wayside simulators, network traffic generators, an environment controller, and data loggers. The simulation can include virtual PTC-equipped trains and WIUs within a field test to perform stress-testing of PTC communication networks, for example. The test bed is instrumented for test monitoring and to achieve a controlled test environment.

The ITC system will be upgraded to include a wayside status relay service, while it and ACSES II will both be upgraded to include an additional control point on the test track.

TTCI remains committed to supporting the industry in its efforts to meet the requirements of the Rail Safety Improvement Act of 2008, and is continuing its research and testing related to PTC.