As I recall, the New York Ontario & Western was the largest rail abandonment up to its time. I think the Rock Island was also large but many of its lines were purchased by other lines and kept running. The Milwaukee Road's Pacific extension abandonment was another very large abandonment by a single like. The Rutland was also a large abandonment.
As for narrow gauge, the Grand Narrow Gauge Trunk was abandoned - much of it was became the SSW in standard gauge. The eastern end was the TC&StL. I do not recall off-hand what become of its lines.
Doug vV