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Thursday, August 11, 2022

Measurements on the move

Network Rail is not just the infrastructure controller. It manages more than 100 rail vehicles for roles as adverse as weather mitigation and incident response. Although part of this fleet was inherited from British Rail, in recent years the company has made a significant investment in its infrastructure monitoring trains, operated by DB Schenker. Amongst them is the multipurpose New Measurement Train (NMT), converted from an HST set and maintained at Heaton and Craigentiny depots. Rail Vehicle Engineering Limited looks after the other vehicles.

The infrastructure monitoring fleet is based in Derby as are the 32 on-train technicians, ten support staff and six engineers who maintain and calibrate the instrumentation. This department is part of Network Rail’s Asset Management organisation which is also responsible for infrastructure maintenance.

Brendan Rice, Examination Resource Manager, explains that the fleet records 250,000 miles of infrastructure each year using 13 recording platforms that provide the 11 streams of data shown in the table. In addition, all vehicles are fitted with a Real-Time Positioning System (RTPS) that ties all this information to the exact mileage and line (ELR). RTPS was developed by Omnicom Engineering Ltd and uses differential GPS, inertial guidance and map-matching when GPS is unavailable, such as in tunnels.

Other vehicles in the fleet supplement the instrumentation coaches – these include driving trailers and staff coaches as well as brake runners for those coaches that cannot be braked because of the instrumentation fitted to them.

Ensuring the trains cover the network at the required frequencies is a complex logistical exercise, demanding a three-year planning programme. The NMT and ultrasonic trains (UTUs) work to a frequent cycle and have Working Timetable paths, whilst other trains operate to a six-monthly rotation, with plans made on a short-term basis. Whenever possible, measuring platforms are combined to form a single rake. As an example of this, the EMV runs with the TRU in third-rail areas. The same applies to the SGT and UTU as both have to operate at night. This is because the UTU is limited to 30mph and the SGT’s gauging laser is ineffective in daylight.

Brendan explains that track geometry and rail defect measurement are an integral part of the mandatory track inspection process whereas other data streams are used to complement the current maintenance regime and assess infrastructure capability. They also provide asset information to whoever needs it within Network Rail, for example gauging details and video surveys for project managers. He reveals that a number of other measurement systems are currently under development, with Network Rail maintenance assessing their suitability. One such is video inspection of switches and crossings.

The trains provide a platform to obtain both current and new asset information requirements in a cost effective manner. A requirement to survey the third-rail network was completed within six months by fitting a measurement system to one of the vehicles at a fraction of the cost of manual surveying. When this proved successful, the EMV has now been permanently fitted with the instrumentation, measuring third-rail position and identifying hot spots with thermal sensors.

Shining light into the darkness

The extent of the network covered by the infrastructure monitoring fleet is illustrated by the Structures Gauging Train’s (SGT) run to Thurso – 704 rail miles from London. On-train technician Paul Walters describes the features of its individual vehicles –

• driving trailer: infrared camera and infrared headlights

• Structure Gauging Coach (SGC): LaserFleX gauging unit, standby white light system, TrackScan, cant and curvature measurement, real-time positioning system

• instrumentation and generator coach: workstations to monitor system outputs and a generator to power equipment

• crew and generator coach: messroom and generator to power equipment

• brake runner coach: provides additional braking force as the structure gauging coach is unbraked

• locomotive: normally a Class 31 fitted with an infrared camera and infrared headlights.

The structure gauging coach, a product of British Rail, has an unusual look: it is almost cut in two to accommodate the white light gauging system with its 23 halogen lights – these project a shielded narrow beam of strong light around the train. The reflection of this light from nearby objects is recorded by cameras and fed into software that triangulates distance to produce a gauge profile and could only be used during the hours of total darkness. It was replaced in November 2009 by the LaserFleX system produced by Balfour Beatty Rail Technologies, although it remains available as a backup.

LaserFleX operates on a similar principle to its predecessor but uses an array of 45 lasers which produce 855 micro beams, the reflections from which are detected by eight cameras mounted alongside. As well as being more accurate, it is also less sensitive to ambient light so performs acceptably in twilight and is largely unaffected by light pollution at stations. Although it can operate at speeds up to 50mph, a minor constraint is that, for safety reasons, the LaserFleX unit is not used below 7mph. Thus whenever the train stops there is a short length of unrecorded infrastructure.

Also mounted on the SGT is a TrackScan 6 unit supplied by DeltaRail which measures the exact distance between adjacent tracks. This is a rotating laser that scans beneath the coach, calculating the distance using the phase difference of the returning laser light.

Position perfect

To provide points of reference, all these systems are supplemented both by cant and curvature measurement and the Real-Time Positioning System (RTPS). The former takes its input from linear vertical differential transformers, accelerometers and gyroscopes on the bogies of the SGT. Twin tachometer encoders supply data on wheel movement for times when a GPS signal is not detected; for this reason and to reduce wheel wear, the SGC is not braked. A rigid coupling between the driving trailer and the SGC is used to provide enough inter-vehicle spacing for the LaserFleX system housing. The SGT also has a Mk2 coach to provide additional braking force.

RTPS was developed by Omnicom Engineering and is fitted to every infrastructure monitoring train. It uses GPS and inertial guidance to record the ELR and exact mileage. As RTPS cannot be relied upon to detect a change of line, particularly at high speed turnouts, all such changes are logged during the run and if necessary RTPS can be manually repositioned.

An infrared camera provides a video record of the run, with the way ahead illuminated by infrared spotlights – these are used so as not to blind the driver of any oncoming train. The recording assists in post-processing and enables positive visual identification of key features detected by the structure gauging.

During the journey from Thurso, Paul Walters and his colleague Phil Blanksby each manned their monitoring stations. Paul observes the measuring systems whilst Phil produces a log of the run and key features from the video. Typically the train generates around 40 Gigabytes of data per night which is recorded onto a removable hard disc; this is then given to the Engineering Data Centre (EDC) on return to Derby. Unlike with track measurement, there is no requirement for real-time reporting as the train is providing a measure of infrastructure capability. Paul is clearly impressed with the technology, recalling that it took him around three hours to gauge a bridge when he was a surveyor. In contrast the SGT does this in the time it takes to pass under it at 50mph!

Back at the ranch

At the EDC, John Dixon, Examination Data Specialist (Gauging), explains how the SGT data is processed and distributed, requiring the three streams – gauge, cant & curvature, and geographical position – to be combined using a common clock. These are then composited into five-metre blocks using the closest approach gauge. During data editing, the local copy of the National Gauging Database is cleansed of those structures no longer present and any new ones added. Network Rail’s gauging engineers are then notified of tight structures. Vegetation clearance prior to SGT runs is important as otherwise it might not be possible to obtain accurate profiles.

It has been determined that structure gauging should be assessed every four years and that the driver for this was track – rather than structure – movement, particularly through curves. In recent times Network Rail has been criticised by the ORR and train operators for not meeting this target. However the steep upwards line on the graph next to John’s desk shows that significant progress is being made towards this requirement; it will be achieved next year.

The processed data is supplied to Balfour Beatty Rail Systems in Matlock – maintainer of the National Gauging Database (NGD), updates of which are provided to Network Rail’s gauging engineers every two months. It is they who can then determine whether new rolling stock meets gauging criteria. This requires more than just the structures data provided by the EDC as an assessment of the vehicle’s dynamic gauge is needed. This involves the use of ClearRoute software, developed by Balfour Beatty Rail Systems, which considers static gauging data for the route, the vehicle’s kinematic envelope and track dynamics to confirm that the new stock will have sufficient clearance before it is allowed to run.

Something old, something new

The structures gauging coach, built by BR in 1981, is one of the oldest in Network Rail’s infrastructure monitoring fleet. But its on-board measuring systems are state-of-the-art, enhancing its ability to gauge infrastructure on the move. That investment is already proving its value. Coupled with effective data systems, it will ensure that gauging information is at the fingertips of Network Rail and its customers when they need it.

Article courtesy of the rail engineer magazine.

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