As envisioned in Gateway, Penn South is an entirely new expansion of “Penn Classic” by a block to the South. This would require the condemnation and demolition of a block-and-a-half of Midtown Manhattan, including high-rise buildings and a historic church. Its high costs do not justify its benefits.
Penn South is necessary only because Penn Station is being operated as a terminal, but more than that, building Penn South ensures that Penn Station can only operate as a terminal for the foreseeable future; if the goal is eventually to have through-running at Penn Station, then Penn South makes that harder. Trains would continue to get stuck in traffic jams, and passengers would still have to deal with a packed and unpleasant station. Adding hundreds of thousands of commuters to the already packed concourses at Penn Station is an additional recipe for chaos.
By investing instead on improving service at Penn, Penn Station South becomes unnecessary. RUN does this by implementing through-running at Penn. It is more efficient than terminals and allows more trains to move through the station faster and with greater safety.