79 MPH should be good for passenger cars and a couple of double stacks or auto racks.
I thought that pure freight trains run at 59 mph. Don't know about combo freight/pax though.
There is quite a lot of 70 mph freight territory out there.
59 mph goes back to the same ICC decision that gave us 79 mph. It works this way: A line without automatic block signals or some other form of automatic control system is not allowed to run passenger trains at speeds of 60 mph or faster or freight trains at speeds of 50 mph or faster.
The 79 mph rule is based on lines with signals but without some form of automatic train control, automatic train stop, or cab signals can not run trains at speeds of 80 mph or faster. I think that rule is all trains, that is both freight and passenger.
There rules are entirey separate from and unrelated to the FRA track classes.
What it says is that no matter how good your track is, if you do not have signals your speed limits can be no more than 59P/49F. This is the maximum speed limit and the reason for it on the line that was used by the Sunset East between Flomaton AL and Tallahassee FL. I do not know the actual numbers, but this would also be the ceiling on any speed limit set on the line used by the Vermont train north of White River Jct.
By the way, if a line has a 70 mph speed limit for freight, that means the track class defined by the FRA has to be good enough that a passenger train speed limit of 90 mph coud be set. However, without the required signal and train control system in place, 79 mph is the maximum no matter how good the track.