The biggest was the Wheeling & Lake Erie. The NKP absorbed lots of other smaller lines. The best place to get this information is the Nickel Plate Historical Society at http://nkphts.org/.
I'm seeing this:
10-16-1964 NYC&STL (New York, Chicago & St.Louis - 'Nickle Plate
Road') merged into N&W, WAB (Wabash) & P&WV (Pittsburg & West
Virginia) leased by N&W, and N&W gains control of AC&Y (Akron, Canton
& Youngstown).