It slows trains down because the existing speed limits already have a significant margin of safety built in so the engineer can exceed them by small amounts and still remain safe.  With PTC such fluctuations over the posted limit will trigger an overspeed alarm with the result that crews will check their speed several mph below what is authorized.  PTC does not increase capacity except possibly on low density lines that were previously not signaled.  ACSES does not actually handle any sort of anti-colision duties.  Those are covered by the legacy fixed block cab and wayside signaling system.  On non-ACSES routes the PTC unit is attached to the wires leading to the wayside signal.  If reliable enough this can provide the same functionality as cab signals, but at a far higher price point. Just because wireless data is involves does not mean PTC is the same as moving block or CTBC.  Finally because of the reliance of wireless links, PTC systems are prone to disruption just like your cell phone or WiFi or digital TV signal.  It is a whole raft of components, all with their own failure rates that will require contingency operations when they stop working.  If those systems were never installed those instances of degraded operations caused by PTC and only PTC would never be encountered.