Track speeds:
I collect old employee timetables, and also have some information of more recent vintage. (3 to 4 years ago, quite a few employee timetables were available on a public web site if you knew where to look.)
One thing that is very important in understanding railroad speed limits: Just because a line has a speed limit of 79 mph, 90 mph, or whatever, does not mean that the train pulls out of the station, gets up to full speed, and stays there until it slows to a stop. Most lines will have numerous curves restricted to lower speeds than the line speed. There will be junction areas, crossovers, end of sidings, etc., with much lower speeds, commonly 25 mph or less.
There was also the factor that premier passenger trains were always operated by engineers that were at the very top of the seniority roster. These men, and they were always men at that time, knew their territory like the back of their hand so they knew how fast they could safely go an all points on their territory, regardless of the numbers in the timetable or posted along the track. Thus, if behind schedule, as a matter of pride, both theirs and the company's they would fudge the speeds as far as they dared to get the train back on time.
The first 100 mph allowable speeds go back to the 1930's with steam. The Milwaukee Road between Chicago and Minneapolis had this limit over a considerable part of the distance, and the line was mostly double track as well. This is the route used by the EB today. The line is mostly single track with a speed limit of 70 mph.
The Santa Fe was also early into the 100 mph speed limit game. Don't when it began, but list of allowed speeds for steam engiens by engine number could be found in employee timetables. There was both a 100 mph list and a 90 mph list. (Slower speeds also.) Much of the route used by the Southwest Chief permitted 100 mph though at least early 1960's.
The Union Pacific line across Nebraska and Wyoming allowed 90 mph all the way up to and including early Amtrak.
There were numerous other lines with 90 mph or 100 mph speed limits, as well. Many of these higher speed limits went down with the advent of the ICC (predecessor to the FRA) rule that required very specific minimum additions to the signals to run "80 mph or faster" and the most basic form of signals to be in place to run "60 mph or faster" These regulations are the father of the common 79 mph speed limit, and on some lines, such as the trains in Vermont and the Flomaton FL to Tallahassee FL secton of the Sunset-East route, their 59 mph speed limit.
Some specifics: In the northeast: Most of the Northeast Corridor had a speed limit of 80 mph during Pennsylvania Railroad days. This is now of course 110 mph and higher.
The New York Central had a speed limit of 85 mph for most of the route used by the Lakeshore Limited. The Twentieth Century maintained its schedule by having almost no stops, and of course being given absolute priority. The speed limit is now 79 mph. In addition, up until the 1950's the line had four main tracks throughout, two for passenger trains and two for freight trains. Beginning sometime during the 50's the outside track were taken up piece by piece.
I think the Pennsy limited their speeds to 80 mph. The Broadway Limited matched the 20th Century in time by having a route some 50 miles shorter, and again, being givven absolute priority. They also had four main track out of the east to at least as far as Pittsburg.
The Atlantic Coast Line allowed 100 mph on lengthy segments of their mostly double track main line between Richmond VA and Jacksonville FL. This was still at 90 mph into the early to mid 1960's. It is now 79 mph and most of the second main is gone.
When I see the current day City of New Orleans, I do not even want to think about it. Into the early 1960's both it and the overnight Panama Limited could be depended upon to make it between Chicago and New Orleans in 16 1/2 hours. There was a lengthy section in Illinois that permitted 100 mph, and most of the rest of the route was nominally 79 mph, which was usually observed only if the train was on time. Now it is all 79 mph or less, and the direct passenger route between Memphis and Jackson MS is no longer in use, and maybe not all still in place.
Some routes, such as the route used by the Texas Eagle between St. Louis and Poplar Bluff MO is lmited to 60 mph, but has never been faster.