Supposedly the comment was offered by the man behind the Carson and Colorado, Darius Ogden Mills. He was supposed to have said, after his first inspection trip to Hawley (Keeler) on July 12, 1883,
"Gentlemen, it seems to me that either we have built this railroad 300 miles to long or 300 years too soon."
This quote is from Mixed Train Daily and Beebe puts it in quotes as a direct quote.
David Myrick also refers to this in Volume 1 of his RRs of Nevada book and gives us the exact date, but I don't know of any contemporary sources to Mill's exact comments.
The Bancroft Library at the University of California, has a load of C&C documents from as early as the 1880's. Maybe Kneiss found it there?
Trying to find the original source should get interesting. Since so many excellent historians starting with Kneiss and Myrick have accepted it as truth, it probably is true, but finding the actual source could be fun.