During the government operation of the railroads during WWI by the USRA there were several strategic decisions that were made that resulted in the eventual abandonment of railroads. I know that these included Colorado Midland and Las Vegas & Tonopah. Were there any others?

Were there any significant railroad abandonments prior to WWI?

One here in Ohio that I can think of: The Findlay, Ft. Wayne & Western. It Ran from its name sake city of Findlay, Ohio to Ft. Wayne, IN. It apparently was deemed of little value by the USRA and abandoned.

Kevin

The New Castle and Frenchtown RR was opened in 1831 and abandoned in 1859, replaced by the Chesapeake and Delaware Canal. How many railroads have been replaced by a canal?

The Chesapeake and Delaware Canal was in operation before the completion of the Newcastle and Frenchtown (RR). I think you'll find that the Newcastle and Frenchtown's demise was a result of the completion of the Philadelphia, Wilmington and Baltimore RR, and the changing traffic patterns it caused.

Still an early abandonment.

I went looking for information and found one website that says that the line lost out to more continuous rail routes to the west. Part of the line survived. See:
http://jlmeek.com/ncf/
http://en.wikipedia.org/wiki/New_Castle_and_Frenchtown_Turnpike_and_Railroad_Company
http://www.pencaderheritage.org/main/landmarks/phland_p18.html

I posted to another board and there I'm getting some western roads.

The "Granite Railway" in Quincy, Mass., may have been the 1st railroad to be abandoned in the United States of America.
An early common carrier abandonment was the Belmont Plane of the State of Pennsylvania's main Line of Public Works, about 1850. The State built a bypass and sold the old line to the Philadelphia & Reading, who abandoned the parts they didn't want. There are still 9 stone ties near Belmont & Parkside Aves. in Fairmount Park, still remaining where they were last used about 160 years ago!

Let's not forget the Allegheny Portage Railroad...but I guess the that depends on what you mean by significant.

If the survey includes electric interurban railways, I think there were several that came and went fairly rapidly. They might be categorized as "It seemed like a good idea at the time" projects. Also, rail lines related to mining operations were often abandoned shortly after the ore "petered out".

I imagine the folks in Quarryville and Peach Bottom considered it "major" when the Lancaster Oxford and Southern (Little, Old and Slow) was abandoned. But didn't large numbers of those local short lines fail and get abandoned quickly?