Modern rail signal systems are one of the most complex and critical aspects of railway infrastructure. In this Q&A, four of EXP’s rail signaling experts share their perspectives on the evolution, challenges and innovations shaping today’s train control systems. Jon Bisignano, Senior Signal Engineer, Kuljinder Singh, Senior Signal Engineer, Steeve Bissereth, Signal Engineer and Berhane Tadese, Principal Signal Engineer explore the components, technological advances and real-world challenges of designing and maintaining the signal systems that keep our railways running safely and efficiently.

What are the primary components of a modern train signal system, and how do they work together to ensure safe and efficient rail operations?

Berhane: During my tenure at The New York City Transit Authority (NYCTA), I was involved in various projects related to train signaling systems, which provided me with a solid understanding of the key components of modern train control systems. In the process, I learned why the NYCTA decided that communication-based train control (CBTC) is the best path forward for the New York City subway system.  CBTC is a state-of-the-art signaling system designed for metro systems, and it utilizes two-way digital radio frequency communication between CBTC trains, also known as intelligent trains and zone controllers. CBTC systems typically include components such as wayside zone controllers, transponders, transponder interrogator antennas, wayside radios, wayside communication networks and carborne equipment on the train. In addition to these modern systems, the auxiliary wayside system replaced the traditional relay-based interlocking with a solid-state interlocking.

Solid-state interlocking systems provide input to CBTC to determine the movement of authority limit information. For this reason, solid-state interlocking is treated in design as one integrated modern train control system. The system also provides interlocking functions in a vital and fail-safe manner. It controls the wayside, like signal heads, automatic electric train stops, interlocking the track switches movements in a safe manner and receiving axle counters or traditional track circuits as input for train detection, as well as providing complex signaling design with proven software logic. Something I find interesting is that the high-resolution train location determination can be achieved by CBTC Carborne equipment, independent of an axle counter or track circuits.

Overall, ZC and AWS function together seamlessly to manage safe train movements and display information at a centralized control center for the train operator. In general, the ZC operates by reading passing CBTC trains through a transponder, then exchanging messages via radio between Carborne equipment and Wayside Zone controllers. These interconnected systems enforce safety, provide shorter headways, operational flexibility and lower maintenance cost due to less field equipment. The other subsystem, CBTC- Automatic Train Supervision (ATS), provides control through monitoring and managing operations from a centralized location known as the rail control center or other locations in the NYCT subway system where remote workstations are connected to the CBTC-ATS system. The ATS performs other capabilities such as train tracking, automatic dispatching based on schedule, and many more advantages for running complex metro track networks that provide optimal throughput, control and flexibility.

How do engineers account for factors like train speed, track layout and train density when designing and implementing a train signal system?

Steeve: The track and civil configuration, train characteristics and planned operations are primary inputs to the design of any signal system. When talking about improving railroads through digital tools, we always talk about improving their safety as well.

In today’s world, software used for designing signaling systems can be developed in extreme detail. Station architecture, specific functions for wayside controllers and many other details can be simulated. Whether we’re implementing a cab signaling system, fixed block, automatic train control, PTC or CBTC systems, today’s simulators can replicate most aspects of the track layout, train configuration or wayside equipment. Testing with simulators also offers flexibility to create diverse scenarios in a controlled environment, reducing the risk of real-world disruptions and offering the opportunity for efficient training without impacting operations.

When designing and implementing a train signal system, these are some of the key factors engineers account for:

  • Track topology – The complexity of the track layout for a given railroad, including turnouts, crossovers, curves, sidings, single/double tracks would greatly impact signal placement and design.
  • Train speed – Operating speeds are generally determined by required sight distance and signal aspect design to allow for safe braking distances. Visibility conditions, stopping distances and track curvature are usually considered.
  • Signal placement – Signal locations are placed strategically to provide enough warning time to train operators for them to react accordingly.
  • Communication systems – Play a critical role by enabling real-time data transmission between the train and wayside equipment. They also play a vital role in relaying wayside information such as accurate train positioning, wayside failures and more to a centralized command center or local dispatcher office.
  • Maintenance accessibility – Design for easy access for inspection, repair or replacement of equipment.
  • Regulatory compliance – Engineers must adhere to all relevant standards and regulations.

What role does modern technology play in enhancing the effectiveness and reliability of train signal systems?

Kuljinder: Modern technology plays an important role in enhancing the effectiveness and reliability of the train signal system. It makes the signal system safer and more efficient.

Digitally-based signal systems make features available that were not practical in earlier relay-based systems or signal systems before that.

  • The control logic is more reliable because it is electronic rather than electromechanical and because the logic is often implemented in redundant sub systems. Therefore that failure in either sub system does not impact system operations.
  • Real-time diagnostics are available locally and remotely improve failure response time for digital and physical equipment.
  • More detailed recorded diagnostics improve the analysis of railroad signal system and non-signal system events
  • Pre-installation testing, installation and commissioning is more efficient with digital systems as opposed to relay-based train control. The near inevitable logic changes required when an interlocking is upgraded or modified are done in software rather than hardwire, which is much more efficient.
  • Features available on some digital systems are automatic dispatching changes, interfaces to crew assignment software, anti-bunching, indicating on operating panels the trains that are running late or automatic routing modes based on anticipated track outages.
  • The key functions of Automatic Train Supervision (ATS) are train monitoring, automatic route setting etc.

In the US, NY-MTA implemented CBTC on a few lines like the Canarsie Line, Flushing Line, Culver Line and Queens Line, which is partially done. The LIRR and MNR have implemented PTC.

In what ways do train signal systems incorporate mechanisms to mitigate the risks of signal failures or malfunctions, ensuring the safety of both passengers and railway personnel?

Jon: Signal systems are typically divided into “vital” (safety related) and “non-vital” (not safety related) subsystems. The implementation, maintenance and testing of these two subsystems are different.

A vital system must be designed to “fail safely” after a single point failure of any component or must meet a specified mean time between hazardous events. Portions of vital systems may also be evaluated by an Independent Safety Assessor. FRA governed railroads must also comply with “Subpart H—Standards for Processor-Based Signal and Train Control Systems” rules for verifying safety, and the rest of the FRA requirements.

What are the challenges + rewards you experience as a signal engineer?

Jon: The biggest challenges include integrating a new signal system project into an existing railroad signal system. The objectives include doing so with minimal service interruptions, meeting safety criteria and doing so efficiently from the perspectives of cost and schedule.

The rewarding aspects include working with diverse technologies and their associated experts. A typical signal project might include trackwork (work on the rails and roadbed), traction power distribution, utility power distribution, electromechanical devices, pneumatic devices, computer-based devices and communication equipment. Design inputs typically include input from service delivery people, maintenance people, track engineers, and sometimes architects, structural engineers and independent safety assessors. The people on signal projects include engineers from different disciplines, from within EXP and from outside EXP.

The greatest reward is being a part of delivering a signal system which improves the reliability, safety or performance of the railroad you are working on.