Economic and Operational Benefits of Rail Grinding on the MBTA Green Line
By
Mark O’Hara and Joseph A. Oriolo • July, 2006
Necessity, it is said, is the mother of invention. When the Massachusetts Bay Transportation Authority (MBTA) introduced a modern low-floor vehicle into operation on its century-old Green Line, a great deal of inventiveness was required to make it work.
The MBTA began operating a low-floor car on the Green Line in 1999. Compatibility with the existing track conditions presented challenges from the beginning. In order to make the low-floor car accessible to mobility-challenged passengers, it was designed with a radically different technology of the truck assembly under the low-floor section of the center truck. This technology utilized stub axle wheel mountings, or independently rotating wheels, as opposed to solid axle wheel mounting. Due to this technology, the wheels of the center truck do not "steer" like those on solid axles in conventional truck assemblies. Consequently, the center trucks on the low-floor vehicles proved problematic.
The low-floor car experienced several derailments shortly after its introduction. These derailments resulted in suspension of low-floor car operations and prompted an investigation into the dynamic behavior of the car, particularly focusing on the center truck.
The low-floor car originally was fitted with the same type of wheels that were used on the Boeing and the Kinkisharyo Light Rail Vehicles used on the Green Line. Among the key features of this wheel are a 63-degree flange angle and a 3/4-inch-deep flange. This same wheel profile had been on the PCC cars, predecessors to the LRVs, which were running on the Green Line long before it was even known as the Green Line. The imprint of this 63-degree wheel flange angle and 3/4-inch-deep flange was worn into every rail and switch on the Green Line (see Figure 2). Contact with the worn-in shelf initiated contact stresses on the corner of the flange tip and reduced the effective contact angle, which increased the potential for wheel climb.
The contact surfaces of new wheels and rails have separate, unique geometries. These contact surfaces are designed or engineered to be different in specific ways for a number of reasons. One of the most important reasons is to help control the path of the wheel as it travels along the rail. As wheel and rail contact surfaces wear over time, the relationship between the two surfaces becomes “conformal.” The geometry of both surfaces begins to look exactly like the other. This is a very undesirable condition, as the ability to help control the path of the train wheel as it travels along the rail is lost.
