Riders on LIRR's Ronkonkoma branch faced rush-hour delays Wednesday and this morning as crews try to finish building a signal box from scratch after one was crushed in Sunday's freight car derailment, the commuter rail said.

Workers, guided by the original 1987 paper plans for the signal case, toiled into the evening Wednesday to string 500 wires and dozens of other components inside the signal case. They worked in a Garden City railroad facility, hoping to truck the finished box Thursday to the derailment scene, just west of the Wyandanch station.

"Unfortunately, it's not the kind of thing you can duplicate overnight," said William Hogan, assistant chief officer of the Long Island Rail Road, as he watched the work. "The idea is to try to duplicate what we had . . . and put this thing together as quick as we can."

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Crews of six or so have been rotating in rebuilding around the clock, but still, Thursday will be the fourth day of abnormal schedules on the branch.

Besides delays of up to 25 minutes, two eastbound trains will be partially canceled this morning and bus service provided, the LIRR said.

The 5:11 a.m. train from Penn Station will end at Farmingdale, but riders headed for stations east of Farmingdale must get off at Hicksville, where buses will take them for the rest of their trips. The 7:54 a.m. train from Deer Park to Riverhead will originate in Ronkonkoma at 8:10 a.m.; Deer Park, Brentwood and Central Islip customers will be bused.

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If all goes well, the box will be installed Thursday, said LIRR spokesman Aaron Donovan.

The case, the size of two big refrigerators, also houses all circuitry for three crossings and governs the railroad's speed control system, which is why the LIRR has been operating under reduced speed near the site. It had 30-year-old components no longer in production, Hogan said.

The New York and Atlantic Railway car that derailed was carrying lumber. Officials at the Queens-based company could not be reached Wednesday.