INTRO: Richard Hope reports on a project to validate the non-vital automatic train protection overlay developed by GE Harris, using radio data links and GPS location. Now in the second of four phases, it is on course for completion by December
AFTER A DECADE of circling inconclusively around the Advanced Train Control System project before deciding that it could not be commercially justified, simpler and cheaper ways of preventing collisions and enforcing speed limits are being sought by North America railway operators.
One such is Harmon’s Incremental Train Control System, which is being trialled by Amtrak on a section of route which it owns west of Kalamazoo, Michigan. This was described in RG 8.96 p496.
The principal rival to ITSC is Positive Train Separation. PTS is currently being tested on 1 360route-km in the western states of Washington and Oregon owned by Burlington Northern Santa Fe and Union Pacific, including sections over which UP operates trains on BNSF tracks (map, below). Amtrak also operates on these tracks.
The BNSF-UP partnership selected the Pacific Northwest as the pilot area to improve safety, establish interoperability and prove that PTS will function with all train control systems currently used by both companies. This area includes joint trackage between Portland and Tacoma where UP freight trains operate over BNSF tracks and under BNSFdispatching control, requiring both companies to be interoperable.
The territory also includes all train control systems used by both railways throughout their respective networks, enabling system-wide implementation without significant modifications to system design. It includes Kelso, Washington, the site in November 1993 of a destructive head-on collision between a BNand a UP train in which five crew members died.
Following a decision by two of the five major US railways to co-operate in the testing of automatic train protection, GE Harris was contracted as systems integrator for the Proof of Concept of PTS. This is currently in progress and due for completion in the first half of 1998.
Non-vital overlay
PTS does not replace or interfere with the existing method of controlling trains. While most routes covered by the BNSF-UP Proof of Concept tests are equipped with conventional CTC, they also include 190 km under track warrant control of which 105 km also has automatic block signals, and another 100 km of double track with block signals.
Track warrants are movement authorities checked by computer logic before being transmitted verbally by the dispatcher to the train crew. PTS transmits these track warrants digitally and displays them in the cab.
Whether signalled or not, control of all BNSF routes is exercised from that company’s Network Operations Centre at Fort Worth, Texas. UP routes come under the Harriman Dispatching Center in Omaha, Nebraska. So far as the dispatchers are concerned, they will continue to issue movement authorities by CTC signals or voice messages in the normal way, but PTS adds some new features to help them.
The three key elements of PTS can be seen in Fig 2. The first, located at Omaha or Fort Worth, is the central PTS computer or server for each rail operator. These servers communicate with the existing computer-aided dispatching systems and the computers handling management information systems. Every time a movement authority is sent to a train through the signalling or a track warrant, the PTS server picks this up and transmits it directly to the locomotive through a digital radio link, the second key element.
The movement authority will include speed restrictions, with associated details of any work being carried out on the track which require confirmation from the foreman in charge of the site that it is safe for the train to enter the work zone.
The third element is the computer installed on locomotives equipped for PTS. This uses global positioning satellites (GPS) augmented with differential correction signals, inertial sensors and odometers to fix its location.
Geographical track data such as gradients, curves, control points, track speed limits and train characteristics are resident in the on-board computer, allowing brake performance to be calculated on the locomotive. Train characteristics are obtained automatically from the railway’s Management Information System computers.
The locomotive computer now has all the information needed to perform its primary PTS function of preventing exceedance of a movement authority, or speed limit violations. It does this by calculating braking curves to the start of reduced speed limits or stopping points, and warning the driver if he or she fails to keep within the curve. If visual and audible warnings are ignored, a full service brake application is made automatically and the power is shut down.
Note that classic automatic train protection systems, including those found in Europe and Japan, normally function as a local extension of the existing signalling. In the event of a danger-side signalling failure due, for example, to a technician’s wiring error during repairs to an interlocking, the ATP is likely to replicate the same error aboard the train.
Because PTS is an overlay, which draws its movement authority data direct from the dispatcher’s computer and is not otherwise connected to the CTC or automatic block signals, it may continue to enforce movement authorities even in circumstances where lineside signals are displaying a false clear aspect.
Additional protection
The radio data link is also used to download to the server every minute the locomotive’s position, as established by the Location Determination System (LDS). The dispatcher is therefore warned if any PTS-equipped train runs past a danger signal or otherwise exceeds its movement authority. He or she is also alerted to the fact that a fresh movement authority needs to be generated when a train is nearing the end of the existing one.
The server also acts as a further check on the safety logic of computer-aided dispatching, which is meant to ensure that no conflicting orders are issued by the computer-aided dispatching system or the dispatcher. Before the server transmits to the locomotive a new PTS Enforcement Authority (PTSEA - pronounced ’pizza’) it has ensured that there is no conflict with any rules the server is responsible for monitoring. This is particularly useful in dark (unsignalled) territory, where there are no track circuits to detect trains and thus provide automatic signal protection.
All these factors clearly enhance overall safety, even though PTS is non-vital.
Four test phases
The first phase of the Proof of Concept was completed successfully in November 1996. The purpose was to check out all the hardware and communications through thousands of permutations in a protocol designed to exercise every element. All the required demonstrations were met satisfactorily.
Two BNSF and two UP locomotives were involved in these tests, which took place on BNSF’s Harbor Line and UP’s Kenton Line. PTS commands were transmitted by digital radio from the servers in Fort Worth and Omaha to the respective companies’ locomotives, which reported their positions back to the PTS server.
Test trains were correctly stopped by the PTS as they approached their authority limits. Location determination, at this stage of initial testing, relied primarily on track transponders and locomotive odometers.
Release Two tests, completed between March and June this year, added several levels of sophistication:
