MBTA's investigation established that the low-floor car required design modifications, including a different wheel profile, to run successfully on the Green Line. The new, or Interim Wheel Profile (IWP), as it was named, would be phased in over a period of months.

Analysis of the low-floor car derailment problem determined that the IWP, with a 75-degree flange angle and a flange depth slightly longer than 3/4-inch, would be required to provide an acceptable margin of safety for operation of the car. The challenge was how to integrate this new wheel profile into an existing system, which was worn by the “old” 63-degree wheel profile.

Rail Wear Shelf
In addition to the modified wheel profile, and other vehicle-related modifications, the low-floor car investigation concluded that the track infrastructure would need to be upgraded to successfully operate the car. Of particular significance was the requirement to eliminate any existing side-worn “shelf” at the gauge face of the rail.

The “shelf” condition, which was the imprint of the 63-degree wheel on the side of the rail head, was widespread on the Green Line, especially in curved track. Although it was a natural consequence of long-term train operation and not considered a defect until it reached a defined magnitude, the low-floor car was found to be unusually sensitive to this type of rail wear. This sensitivity and intolerance was determined to be the cause of several derailments of the low-floor car and, therefore, was an unacceptable condition.

The shelf imprinted on the rails of the Green Line by the 63-degree wheels of the existing LRV fleet would also create severe wheel wear problems for the IWP. The 75-degree flange angle and longer flange of the IWP would be totally incompatible with the existing wear pattern. In addition to creating an environment in which the low-floor car would be overly susceptible to wheel climb, the shelf would rapidly degrade wheel conditions on the Green Line fleet. The shelf had to be eliminated.

Elimination of the side-worn shelf on the gauge side of the rail was originally thought to require the installation of new rail wherever the shelf existed. If so, rail replacement would be extensive and would cost the MBTA an estimated $2 million on Beacon Street, the Highland Branch and in the Central Subway. On Commonwealth Avenue, rail replacement could not be done because of a pre-existing condition with the concrete ties installed there in the early 1980s. New ties would first have to be installed to facilitate the rail replacement. This would mean full-depth track reconstruction on Commonwealth Avenue to eliminate the side-worn shelf and enable the low-floor car to operate. This reconstruction would delay the low-floor car operation for at least a year, and cost about $11 million.

Vertical Rail Grinding
In the interest of saving the MBTA millions of dollars and avoiding a protracted construction project, the Maintenance of Way Division investigated possible alternatives to full-depth track reconstruction on Commonwealth Avenue and rail replacement on the remainder of the Green Line. No alternatives were readily apparent until Loram Maintenance of Way, Inc., a rail grinding contractor, was approached with the previously untried idea of “vertical” rail grinding to eliminate the side-worn shelf. The contractor conducted tests and determined (although this type of grinding had never been done before) that its equipment, with minor modifications, was capable of grinding at the 75-degree angle required by the IWP.