The most amazing one to me is the Downeaster route north of Haverhill, which returned as a full-fledged corridor after 35-plus years as a freight-only route. Most of the others I can think of were, like the already-discussed Port Road and the C&O route through Peru, IN, essentially long-term/permanent detours that were made necessary when the traditional or direct route was downgraded or abandoned:
-- the Silver Star/Carolinian route between Raleigh and Selma (replacing the old Seaboard line through Henderson);
-- the City of New Orleans route through Yazoo City (replacing the old Illinois Central line from Memphis to Jackson);
-- the Sunset Limited route through Maricopa (replacing the line through Phoenix).
There were also some routes where passenger service went dark for a few years in the '70s and then returned, including the Lake Shore route west of Buffalo, the Adirondack route north of Albany and the Boston & Albany route west of Worcester, plus the Montrealer/Vermonter line north of Springfield.
And there was a period beginning in 1989 when the Montrealer was shifted onto the Central Vermont line from East Northfield, Mass., to New London, CT, which had been freight-only for many years,