Getting signals across to train drivers

Operating railways safely has been of paramount importance since the earliest days of steam locomotion. How to ensure that a driver can not only see but act on a signal aspect has always been top of the list of requirements. Now radio is providing a means of reducing human failure as this story of US experiences shows.

Already today, technology exists to design a system that can start applying a train’s brakes, should a driver fail to obey stop signals. Positive train control (PTC), a system that monitors railroads through a radio network, can help prevent crashes and enhance railway safety in other ways.

Currently used in several pilot programs, PTC systems are required to be implemented under the US federal Railroad Safety Improvement Act 2008 (RSIA). Antennas will be a critical component to the PTC system.

Transport officials look to technology for further gains in rail safety - a way to correct the human error now responsible for most crashes. The US National Transportation Safety Board has had PTC on its ‘most wanted list’ since 1990. Some experts say the technology could have prevented a crash in California in September 2008, for example, when the driver of a commuter train was reportedly text messaging and failed to observe a signal.

The commuter train slammed into a freight train northwest of Los Angeles, killing 25 people and injuring hundreds.

PTC is a flexible system of monitoring train data, including location, speed, track information and equipment functioning. Information, often gathered with the help of a GPS, is used to warn train operators about safety hazards.

If necessary, the PTC system can automatically slow or stop trains. Information is sent from the train to antennas alongside the track where the data is relayed through another wireless system to the dispatch control centre.

According to the Federal Railroad Administration (FRA), a variety of components, ranging from the digital data network to GPS systems, to onboard and dispatch computers and throttle-brake interfaces, are needed.

The term ‘positive train control system’, according to the FRA, is a system designed to prevent train-to-train collisions, over-speed derailments and incursions into established work zone limits and the movement of a train through a point left in the wrong position.

Systems similar to PTC are already in use for controlling air traffic. Nine different railway companies have PTC pilot programs in the US, including an Amtrak route between Chicago and Detroit and a Chicago area Metra commuter line.

Implementing the system nationwide in accordance with RSIA will require installing antennas in most railway systems. Interoperability is also essential to developing a nationwide PTC system to allow different railroad companies to communicate and share tracks.

Several wireless communication links are involved in a typical PTC system. A GPS antenna placed on top of the locomotive receives location information which is passed onto the core PTC module. The module interfaces with the rest of the control systems in the locomotive. It also collects other pieces of information, such as speed, engine performance, driver awareness, etc, combines it with location information from GPS and sends it over a wireless communication link, through an antenna placed over the roof of the locomotive, to the control centre.

Specially designed antennas are placed at regular intervals along the track to provide a continuous data link with the train. Data received by these antennas is then funnelled over to the control centre, either through a wire-line connection or through a point-to-point wireless backhaul link.

Data from the control centre is sent back to the train via the same route.

All these links require high performance and robust antenna systems to enable accurate and timely transmission of appropriate data. Most of the antenna systems in PTC get deployed in harsh environments, particularly those on locomotives which experience heavy vibration and temperature variations.

In addition to mechanical robustness, the antennas need to be designed to optimise coverage and minimise interference.

GPS antennas on locomotives face significant RF interference from noise generated by the locomotive engines and from other data link antennas, particularly those operating in the 212-222 MHz range that has been dedicated to PTC.

For the PTC system to work properly, the GPS antennas need to be designed with appropriate filtering and the trackside antennas incorporate a pattern designed to provide continuous uninterrupted coverage over the tracks.

To meet the demand for these highly specialised antennas, companies such as PCTEL have designed various wideband/multiband transit and base station antennas. Not only are they built to precise electrical standards, they also have to pass in-house environmental testing for extreme weather conditions and high vibration.

In the US, train use and rail traffic congestion are predicted to rise, putting even greater pressure on the need for PTC systems and the demands of the equipment.