High-Performance Floating Slab Track: Design and Construction Improvements Based on Lessons Learned from Prototype Slabs

Abstract

The Northgate Link extension of the Sound Transit’s Link light-rail transit system subway tunnels under the University of Washington must meet stringent vibration thresholds as agreed between Sound Transit and the university to protect vibration-sensitive research labs. To meet the vibration thresholds the design team developed a 5 Hz floating slab track with an ultra-straight rail as the appropriate solution. Considering that such a solution pushes the engineering envelope, a 440-foot prototype section with 5 Hz floating slab track and a 1000-foot section of un-isolated track with milled rail were installed in the University Link tunnels to test the design. The performances of both prototype sections were evaluated during pre-revenue service and revenue service of the University Link. Based on the performance evaluation, the design of the floating slab track was refined, and the construction tolerances were tightened. The design refinements focused on improving vibration isolation performance, including stiffer rail fasteners, slightly softer support pads, and slightly heavier slabs. The construction specifications included limits on rail undulation consistent with European standards for high-speed track, EU Type 1 rail welding standards, and implementation of a frequency response pass/fail criteria on the installed slabs. This paper provides the details of the design changes, the construction tolerances, and preliminary performance results.