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In the Hudson Highlands, where the cliffs jut straight up from the river, Metro-North's Hudson Line tracks sit on a thin rocky ledge that was blasted from the rock more than a century ago.

This scenic section of track just north of Peekskill is prone to rock slides and washouts and the tracks are monitored by a pair of thin wires that measure geologic shifts, one little slip at a time.

When the rock under the tracks falls away into the river, little by little, as it occasionally does, an alarm sounds and track gangs are quickly dispatched to shore up the rails by adding stone.

Now, as part of the Metropolitan Transportation Authority's Capital Program, an $8.6 million effort is underway to stabilize for decades to come.