The replacement of a 74-year-old bridge in Westbury next weekend could cause major delays for at least 40,000 Long Island Rail Road customers -- and many more if the National League Championship Series between the Mets and Cubs goes to a Game 6 or 7 at Citi Field.

Five months after the LIRR tore down the deteriorated Ellison Avenue Bridge, which crosses its tracks and carries vehicles and pedestrians, the agency will install a new bridge next weekend. The work will force the LIRR to suspend service on its Main Line between Hicksville and Mineola on Saturday and Sunday.

Customers on the Port Jefferson and Ronkonkoma branches will be most affected, with delays of 40 to 60 minutes expected as buses replace trains between Hicksville and Mineola. The LIRR said other branches, including the Babylon, Far Rockaway, Hempstead, Long Beach, Montauk, Port Washington, Oyster Bay or West Hempstead lines, "will have significantly less impact" and may be good alternatives for Port Jefferson and Ronkonkoma travelers.

"We strongly urge our customers to avoid the Ronkonkoma and Port Jefferson branches during the weekend of October 24-25," LIRR president Patrick Nowakowski said. "We regret the inconvenience, but for safety reasons, the installation of the new bridge requires a complete suspension of train service between Hicksville and Mineola."

The LIRR plans to conduct the work even if baseball playoff games are contested that weekend in Queens, where the LIRR carries thousands more customers to its Mets-Willets Point station. In such a situation, the LIRR said customers should use the Babylon or Port Washington branches to get to Woodside, which will not be serviced by Ronkonkoma trains during that weekend.

The $17.5 million construction project includes erecting a new bridge and installing sidewalks, curbs, fencing and paving. It is scheduled to be completed by June.

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The two-lane Ellison Avenue Bridge, with walkways on both sides, was constructed in 1941. It had fallen into disrepair in recent years, and was consistently rated among the worst bridges on Long Island by state transportation officials. Its demolition in May required a similar weekend-long suspension on the Main Line, which Nowakowski said "worked out well" because of the advanced notice provided by the LIRR.

"The cooperation of our customers was outstanding," Nowakowski said. "Once again we are asking for their help."