NEW YORK — Longtime New York City subway cops have seen copper thefts before, but last week’s heist on a stretch of tracks was particularly brazen: 500 feet of inch-thick cable stripped from the rails in the middle of the night.

The theft that shut down parts of the subway’s biggest lines and snarled the commute for 100,000 people is only the latest example of a troubling trend on the nation’s railways linked to the soaring price of copper.

Copper, which now hovers at $3 a pound on the scrap market compared to about 80 cents a decade ago, is essential to the running of commuter railways, carrying electrical power from substations to the live third rail.