tl;dr: Between the University Street and Westlake stations, the tracks make a 90-degree turn. When taking this turn, riders were experiencing a very herky-jerky ride. The closures and repairs were intended to fix that. The sharp turn had caused the tracks to wear down in the spots where the wheels were grinding against them. This wear-down lead to extra space between the rails that the train wheels would slide back and forth on (causing the train car to rock back and forth). The repair involved replacing the rails at the 90-degree bend.