Well, Phil, not wishing to upset one of my biggest helpers with DCAD, I can find little or no written evidence to support your mentions.
I agree that many American railroads were built in horrible terrain, but that was little to do with the low standard of construction, that was purely down to the way various promoters stole the money from the investors. Many of the slumps from 1840 onwards have been railroad/railway related. More importantly until the Civil War, many American railroads were majorly supported by English Bankers, and many of those caught a cold, both figuratively and literally. The second or third railway in the world to open to passenger traffic was definitely the Baltimore and Ohio, representatives of which were at the famous Rainhill trials when Stephenson's Rocket won the race as it were. But in reality the rapid expansion of American railways was dependent on the Civil War, and then the opening of the Indian Territories.
However, this does not change the fact that the record for 999 was dubious to say the least, and careful modern study of the data suggests she went no faster than 82.5 mph. Interesting though that one of Henry Ford's first record breaking cars was 999 too. However, engines in the UK were certainly influenced by the American Builders, the Atlantic and the Pacific, in particular the K4 from the Pennsylvanian Railroad was particularly influential on the evolution of the A1, 3 and 4 Pacifics of the English East Coast route.
Knowing how proud America is of its speed records, (I am old enough to remember Craig Breedlove and his first attempts on Donald Campbell's land speed record) I would have thought there would be much more publicity around a 100mph 700 mile regular journey, but since even Chicago Museum does not mention it, I am not sure. There is great play made of the 700 mile in 70 hours, but not much more. Indeed if I look on wikipedia, it seems that until about the 1930's there was very little attempt to push the idea of high speed in the States. Certainly no mention of earlier high speed stuff. I am happy to be shown that I am wrong, but so far not found the data.