The higher than 79mph were running on ATS territory as they do even today - and still continue to with 90mph operation on tracks so equipped. Some were possibly running on Cab signaled with enforcement territory too. Quite a bit of that cab signaling survives minus the enforcement. The regulation does not require ATC (whatever that might mean). AFAIR Either ATS or Cab Signal with signal speed enforcement but no stop enforcement is enough to operate above 79mph. To operate over 125mph one requires civil speed and stop enforcement over and above signal speed enforcement in cab signaled territory, and just ATS without the additional enforcements are also not enough for higher speeds. AFAIR there are no operations at higher than 90mph at present which does not have cab signaling with signal speed enforcement.
So, why is Via allowed to do 100+mph where Amtrak is only allowed to do 79mph under virtually identical conditions?
The answer is mostly political, after the Chase MD wreck the hammer came down on Amtrak, arguably the 79mph ruling was an overreaction to a very rare occurence. Via escaped the trickle down effect.
Except the 79mph speed limit long predates the Chase, MD, wreck. The 79mph limit dates to the 1940s (I believe in response to a Chicago-area wreck) while the Chase, MD, incident occurred in the 1980s.
I'm no expert on the history or scope of the 79mph rule but there were lots of trains doing well over 79mph long after the 1940's into at least the 1960's and not all of them ran on ATC territory, the Santa Fe, Super Chief to name just one.
Eric is correct. The Chase accident has nothing to do with 79mph limit. Its origins are back in the 40s.Also the regulation is not specific to Amtrak. It applies to everything that runs on FRA governed rails in the US.
The net change from Chase had to do with the requirement for Engineers to get licensed, and also some of the alcohol and drug test stuff. In addition rules regarding what equipment must be operational on engines before they are allowed to operate on higher speed territory even if the locomotive itself is not capable of operating at such speeds i.e. have fully operational cab signal with enforcement rather than just a warning whistle that can be taped over etc.