CTA chief of staff Nora Leerhsen will become the interim president of the transit agency, taking over after Dorval Carter’s retirement at the end of January. Leerhsen will, for now, take on leadership of an agency that has faced rampant complaints from riders in recent years, with another challenge on the horizon as a massive budget gap looms and lawmakers in Springfield prepare to debate the future of the region’s four transit agencies in a move that could have significant repercussions for the CTA’s authority.