My vote is Lehigh Valley Rail Road Company; March 2, 1869. Operated 1846-1976, name gone then returned due to RR Corporate mergers and splits and equipment still runs on the track. Ran some interesting, to the model hobby, locomotive stock; 2-10-0, 2-10-2 and an 0-6-6-0
Wikipedia has a listing for more details.
It was one of America's politically correct "C.C.R.R." moving much Pennsylvania coal.
[C.C.R.R.= Climate Change Railroad