May 13, 2013

Continuous rail turnouts favour mainline track

Written by  David Davis & Rafael Jimenez
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Tests of a new continuous rail turnout concept, which was recently commissioned on a BNSF line in Texas, has the potential to reduce maintenance costs and improve safety, as David Davis, senior scientist and Rafael Jimenez, senior engineer at Transportation Technology Centre Inc, United States, explain.

LIKE all of the Class I railways in the United States, BNSF's network has thousands of turnouts to diverging passing loops and service lines to industrial facilities which, while only carrying minimal traffic, are essential to its everyday operations.

Despite not being in regular use, each turnout requires around 20 times more maintenance than plain track because of the exposure of its vulnerable components to the forces of every passing train.TTCI-Fig1 2

In an effort to increase the turnout service life as well as improve safety, BNSF, Progress Rail Services and Transportation Technology Centre Inc (TTCI) joined forces to develop a continuous rail turnout concept. As the name indicates, the key feature of the design is the continuous mainline rail through the switch and crossing which aims to improve the performance of the mainline side of the turnout at the expense of the capability of the diverging route.

A prototype continuous mainline rail turnout designed for low speed and low volume diverging traffic was evaluated under 39-tonne axleload traffic at TTCI's Facility for Accelerated Service Testing (Fast), in Pueblo, Colorado in spring 2012 following a two-year development programme.

The switch configuration of the new design (Figure 1) differs from a conventional turnout design (Figure 2) by having both fixed stock rails on the mainline route. The conventional switches have one fixed stock rail and one moveable switch point on each route with both routes having running surface discontinuities on one rail. As the train passes over the turnout, wheels transition from stock rail to switch point on one rail of each route with the moveable switch points both located on the diverging route. Note in Figure 1 that on the continuous mainline rail turnout, one switch point is located on the gauge side of the left stock rail, and the other switch point is located on the field side of the right stock rail.

The continuous mainline rail turnout design is also referred to as a "vertical switch" because it functions by lifting wheels over the mainline rails, instead of providing a gap in the mainline rail for wheel flanges to pass through. This switch is similar to the lift frog design, which has been successfully implemented by North American freight railways. Like the lift frog, this design strongly favours the mainline in terms of ride quality and allowable speeds so is ideal for loops and industrial sidings accessed from the mainline.

Testing

To evaluate the prototype design, the test team conducted a series of proof tests, consisting of mainline and diverging operations, track strength measurements, and running surface wear measurements. Mainline testing consisted of operating 142,000kg wagons at 64km/h over the turnout with approximately 30 million tonnes of passing traffic accumulated. Diverging operations consisted of spotting approximately 275 loaded and empty wagons in a 107m siding at speeds ranging from 3km/h to 16km/h.

TTCI-tabs-1 2The tests showed the prototype turnout performed very well in most aspects. The dynamic forces for mainline moves were quite low, similar to what is measured on open track, while dynamic forces on diverging moves, as Table 1 shows, were acceptable. A comparison was also made with a nearby split switch turnout. Even though the split switch turnout is larger (No 20 versus No 11), the lateral forces were comparable. Maximum vertical loads are somewhat higher for the vertical switch from ramping in the switch, where wheels are raised above the stock rails.

Table 2 lists the measured maximum dynamic loads for mainline moves through the two turnouts with the data split between the switch and the frog on each turnout. Note that in each location the continuous rail turnout produced lower maximum lateral forces. There were also similar drops in average lateral forces. The changes in running surface in the conventional switch and frog generate lateral forces above those seen in open track. Maximum vertical forces at higher speeds should be similar to open track with continuous welded rail. Previous tests of a lift frog by TTCI at Fast showed 95% vertical forces of 47 and 76 kips for lift and railbound manganese frogs respectively at 64km/h.

TTCI-tab-3Testing also analysed stock rail and switch points wear. The switch points, with their non-conformal shapes and short vertical ramps, wore at a much higher rate per million-tonne than the stock rails. Table 3 shows wear rates for each of the four running rails in the prototype switch, with the switch point wear rates two to three orders of magnitude higher. However, the initial wear in the first measurement interval of 0.11 million tonnes was about 50% of the total wear of the gauge side and 90% for the field side points. The wear rate is expected to continue to decrease as the points reach shapes that are conformal to wheels.

Figures 3 and 4 show the switch point wear versus location for three accumulated tonnages. Note the similarities and differences in wear of the two switch points. In both cases, the locations of highest wear rate are at the point ends where the wheel impacts the end of the point as it rolls in a facing point move. Also noteworthy is somewhat higher wear at the top of the point ramps.

TTCI-Fig3 4The effect of running surface profile may explain the higher wear rate (reported as cross section area loss) on the field side point. Both points started with the same rectangular shape with the running surface available to the wheel on the field side point much smaller and perhaps less conformal. In addition, the field side point is a composite structure consisting of three pieces. Shifting of the pieces relative to each other may cause some of the wear reported for this switch point.

Since completing the tests, several design improvements have been made by BNSF, Progress Rail Services, and TTCI ahead of introduction into revenue service, including:

• lateral stiffening of the field side point to prevent gauge widening

• reconfiguration of the switch heels to reduce switch throw effort, and

• reconfiguration of the point guard to increase the available contact area with the wheel and to simplify construction.

A continuous rail turnout incorporating these modifications is now in revenue service on a BNSF line in Texas. While the mainline carries 60 million tonnes of traffic per year, the turnout is only used once or twice a week by a company shipping industrial aggregate, and is therefore an ideal example installation for the new turnout. Since installation in March, the turnout has performed well, with analysis continuing.

The next step for the continuous turnout concept is to receive approval from the American Standards Commission at its meeting with the track structure engineers this month. If this is secured we expect significant interest in the concept from other railways in North America and will work with the supply industry to enable them to develop concepts for market. While we worked with Progress Rail Services on the system for BNSF, VAE has also worked on a concept with a view to working with Union Pacific, but has not yet developed a prototype.

TTCI employees Mr Muhammad Akhtar, Mr Joseph LoPresti, and Ms Beatrice Rael contributed to this article.

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