I suppose we need to make a distinction between types of "nicknames." Some were officially used by railroads, others were not. And then, nicknames were different from official slogans.
For example, the St. Louis Southwestern was the "Cotton Belt Route," the CB&Q was "The Burlington Route," the CMStP&P was "The Milwaukee Road," the CRI&P was the "Rock Island Lines," and the NYC&StL was, as you've mentioned, the "Nickel Plate." These were official nicknames because they were on the railroads' heralds.
Then, there were unofficial nicknames, such as the "Pennsy" for the Pennsylvania Railroad, the "Northwestern" for the C&NW, the "Louie" for the M&StL, the "Great Western" for CGW, or the "Espee" for the Southern Pacific. These didn't appear on any heralds or car lettering, to my knowledge.
Additionally, there were derogatory nicknames, such as "Busted and Maimed" (Boston and Maine), "Great Weedy" (Chicago Great Western), "Old & Weary" (O&W, or New York, Ontario & Western).
Finally, there were what I am calling slogans, such as the CB&Q's "Everywhere West," the Wabash's "Follow the Flag," or the PRR's "Standard Railroad of the World." These appeared on timetables or other publicity.
I don't recall hearing any nicknames for the New York Central, either official, unofficial or derogatory unless it was "the Central." "The Water Level Route" was not a nickname but a publicity slogan -- railroaders or fans would not have been likely to refer to the NYC that way in casual conversation. However, the component parts of the NYC retained some of their original identity at the local level, so that lines in central Michigan might be referred to a "the Michigan Central" or lines in Illinois, Indiana or Ohio as "the Big Four," long after absorption into the NYC system. The Boston & Albany, however, was not just a local nickname but a real identity, since it was the NYC's lessee.
Dr. R. C. Leonard, "Richard Leonard's Rail Archive" ( http://www.railarchive.net/