Why 79 mph - Trains Magazine

In the U.S., maximum speed limit has both a track geometry/defect limit and a Method of Operation limit, whichever is more restrictive is the one that applies:

Track speed limit, as proscribed by 49 CFR 231.9 and 49 CFR 213.307, in mph:

Track Class	Freight	Passenger
Excepted track	10 N/A
Class 1 track 10 15
Class 2 track 25 30
Class 3 track 40 60
Class 4 track 60 80
Class 5 track 80 90

Class 6 track...110 m.p.h.
Class 7 track...125 m.p.h.
Class 8 track...160 m.p.h. 
Class 9 track...200 m.p.h. 

Method of Operation speed limit, as proscribed by 49 CFR 236.0:

 (a) Except as provided in paragraph (b) of this section, this part 
applies to railroads that operate on standard gage track which is part 
of the general railroad system of transportation.
(b) This part does not apply to rail rapid transit operations 
conducted over track that is used exclusively for that purpose and that 
is not part of the general system of railroad transportation.
(c) Where a passenger train is operated at a speed of 60 or more 
miles per hour, or a freight train is operated at a speed of 50 or more 
miles per hour, a block signal system complying with the provisions of 
this part shall be installed or a manual block system shall be placed 
permanently in effect which shall conform to the following conditions:
(1) A passenger train shall not be admitted to a block occupied by 
another train except under flag protection;
(2) No train shall be admitted to a block occupied by a passenger 
train except under flag protection;
(3) No train shall be admitted to a block occupied by an opposing 
train except under flag protection; and
(4) A freight train, including a work train, may be authorized to 
follow a freight train, including a work train, into a block but the 
following train must proceed prepared to stop within one-half the range 
of vision but not exceeding 20 miles per hour.
(d) Where any train is operated at a speed of 80 or more miles per 
hour, an automatic cab signal, automatic train stop or automatic train 
control system complying with the provisions of this part shall be 
installed.