The Shaah Zmanios doesnt work for modern societies because Halachic Noon is differnet in NYC than it is a Philadelphia and things like trains need to have a standard time so noon means the same thing in NYC and Philadephia so there is no confusion
Its not a big deal if the time for Shabbos is slightly differnet for both cities, but it is a big deal if a train is supposed to arrive at noon, you need to know what noon means