Transit Shutdown Under Way for Second Time in History What was once without precedent will now happen for the second time in 14 months: New York City's transit system is going dark. But while the shutdown before Tropical Storm Irene last year began at noon on a Saturday - and the restoration of subway service began before the Monday workday - the suspension of subway, bus, and railroad service this time could prove particularly disruptive. Joseph J. Lhota, the authority's chairman, suggested that the city could be without most of its transit system for two full weekdays. By Wednesday, he hoped, some service might be restored. The subways will begin suspending service at 7 p.m. on Sunday, but some buses could remain on the road until 9 p.m. It takes about eight hours to shut down the subway system, but the bus system requires only six hours to close. Emergency preparations began at the transit agency well before Gov. Andrew M. Cuomo's announcement of the closings on Sunday morning. The authority's hurricane plan calls for service to be suspended if sustained winds reach 39 miles per hour. Thousands of buses and subway cars have already been removed from service and moved to safe locations. Flood-prone subway yards and depots have been cleared, and stations in vulnerable areas, like Lower Manhattan, will be evacuated. The authority said that "critical track-level components" were being removed from beneath the river tubes to protect the materials from the corrosive effects of salt water in the event of flooding. On Metro-North Railroad, equipment was to be removed from low-lying areas like the east end of a New Haven yard in Connecticut and the Highbridge and Mott Haven yards in the Bronx. Some trucks, cranes, bulldozers and other equipment were being moved to higher ground. Plans included bringing trains into Grand Central Terminal for shelter. Some wooden crossing gates were also removed and secured on both Metro-North and the Long Island Rail Road. Riders were warned that a suspension of service did not imply that power would be cut to the third rail or overhead wires. The authority's paratransit service, Access-A-Ride, suspended its outbound trips at noon on Sunday; return trips were expected to continue until 5 p.m. The authority said the Staten Island Railway would continue operations for as long as the Staten Island Ferry was in service, if conditions permitted, so that no riders would be stranded at the ferry terminal. The authority's bridges will close to all traffic if sustained winds reach 60 miles per hour. Required slowdowns are likely be put in place if winds exceed 39 m.p.h. The Port Authority of New York and New Jersey announced that PATH train service would be suspended beginning at 12:01 a.m. Monday until further notice. Operations remained normal at local airports, the agency said, but travelers were encouraged to check with their airlines. Though Mr. Lhota expressed optimism about restoring service by Wednesday, a return to normal operations is likely to come in fits and starts. A little over 24 hours after subway, bus, and rail service was suspended for Tropical Storm Irene, some limited bus service returned. About 14 hours after that, the subways began running. Commuter railroad service was restored on a line-by-line basis. Some of them required substantial cleanup of debris and mudslides, and others waited on power to be restored.