The rail tunnels that connect New York City and New Jersey are 105 years old, and the technique that Amtrak employees have been using to keep trains running through them this frigid winter is hardly more modern.

In essence, it involves a whole lot of whacking.

Three or four times a day, a crew armed with telescoping yellow poles topped with hammerlike appendages rolls through the tunnels hunting for icicles. When the workers spot one dangling from the arched ceiling, they signal for the rail car operator to stop and they reach up and take aim.

A few quick whacks and the Ice Patrol rolls on, having headed off another potential disruption of the tightly choreographed train traffic under the Hudson River. But on some especially cold days — Friday, for instance — the ice forms faster and in more places than the Ice Patrol can handle.

The result can be a disastrous start, or end, to the workday for thousands of commuters who depend on the two single-track tunnels. They provide the only direct passage for trains traveling between Manhattan and points west. Each weekday, about 450 trains operated by Amtrak and New Jersey Transit pass through them.