Interface - The Journal of Wheel/Rail Interaction

GIRDER RAIL

Testing Girder Rail on the MBTA (continued)

The practice of gauge-widening in curves was revisited by the MBTA while testing the NP4aM girder rail. A new set of criteria needed to be developed due to the narrowness of the flangeway with respect to the thickness of the wheel flange. The MBTA used the information produced by the Solid-Works software to determine the optimal gauge and flangeway combinations for the MBTA wheel profile in a 50-foot radius curve. The object was to balance the lateral wheel forces among all four wheels of the truck, with consideration given to the angle of attack and allowing for the fact that the wheelsets would not be radial to the curve.

Williams’ analysis led the MBTA to take a completely different look at how to reduce the angle of attack at both leading and trailing axles, balance lateral wheel forces, combat truck crabbing, prevent wheel binding, and reduce rail and wheel wear. The analysis showed that instead of widening the gauge in tight radius curves, greater benefit, with respect to making the vehicle trucks more radial and reducing the angle of attack, might be derived by actually reducing the gauge.

Because of the narrowness of the NP4aM flangeway and the geometry of the IWP flange, it was considered impractical to use the NP4aM rail on both the inner and outer rails of the curve. Calculations indicated that there would be virtually no free play for the wheelset traversing a 50-foot curve. Any discrepancy from the design gauge would result in excessive flange contact and potential wheel climb. As a result, other options were explored.

The MBTA’s standard ballasted track consists of 115-pound rail on hardwood crossties with Pandrol plates and resilient fasteners. In curves, modified 132 RE restraining rail is bolted to the running rail(s) to create flangeways. The width of the flangeways and track gauge vary depending on curvature. The flangeways are adjustable through the use of a spacer assembly with shims. The MBTA decided that a combination of an inside rail 115-pound/132-pound assembly with an outside rail using NP4aM might provide an acceptable design option.

Field Testing
The MBTA established a test site using a 115-pound/132-pound rail and NP4aM rail combination in a 50-foot radius curve in a turnback loop at the end of the Commonwealth Avenue branch of the Green Line (see Figure 1). The new rail was carefully installed to as tight a tolerance as possible to remain faithful to the design gauge of 56-11/16 inches. The 115-pound/132-pound inside rail set was installed with a flangeway of 1-3/4 inches. The gauge and flangeway dimensions were intentionally conservative with respect to the calculated wheelset free play to establish a baseline for future testing.

Strain gauges were installed at the test site to measure the vertical and lateral forces from passing train wheels. The forces were measured at four points at each of six locations throughout the curve: gauge corner of the inside 115-pound running rail; restraining rail face of the companion 132-pound rail; two points on the NP4aM rail; and the gauge corner and face of the girder flange. Data was collected for every car traveling through the curve. Field observations indicated that while it appeared that this gauge and flangeway combination was beneficial from an angle of attack standpoint, additional improvement could be expected.

Subsequently, the gauge was reduced to 56-7/16 inches and the inside flangeway narrowed by 1/4 inch, to 1-1/2 inches. A random sampling of data from each of the two tests indicated that lateral forces were better balanced at the tighter gauge and flangeway configuration used for the latter part of the test.

While the analysis of the gross lateral force data was somewhat elementary, it demonstrated that the conventional wisdom with regard to gauge-widening in tight-radius curves was flawed. Lateral wheel forces were better managed with narrower gauge and a narrower flangeway at the inside rail.

As a result of the testing done by the MBTA, track design and maintenance standards for double restrained curves (< 100-foot radius) will be revised. The longstanding practice of gauge-widening in tight radius curves will be abandoned in favor of a set of criteria that will serve to balance lateral wheel forces and help control rail and wheel wear.

Mark O’Hara is a Track Engineer, HNTB Rail Systems

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