TRANSIT BIGS BACKTRACK ON SACKED SIGNALMAN By BILL SANDERSON Transit Authority bosses have offered to rehire an employee they blamed for a July 3 train derailment amid conflicting evidence of what caused the crash, The Post has learned. State and federal investigators are still probing the Harlem accident that injured 15 people. The TA had blamed and fired signal maintainer << Ariff Roberts>>  - although witnesses tell conflicting stories of what happened, according to TA documents obtained by The Post. Ed Kehoe, a TA employee who publishes a newsletter for track-signal workers, said several things other than human error could have caused the accident - such as some unknown malfunction in the subways' aged signal and switch equipment. An official of the main subway workers' union said Roberts was made a scapegoat to assure subway riders that the system is safe. "They don't react to the accident. They react to the public pressure and the politicians," said John Mirrione, a vice president of Transport Workers Union Local 100. Kehoe and a Local 100 official who asked not to be named said the TA offered to rehire Roberts with reduced responsibilities and pay. TA officials would not confirm any job offer, and Roberts could not be reached for comment. Jared Lebow, a TA spokesman, said that in their final report, investigators will maintain that Roberts was responsible for the accident. The derailment happened at about 10 p.m. July 3, when a southbound A train was nearing the 135th Street IND station. A track switch suddenly changed under the train's last car - sending the car down two separate sets of track and slamming it into some steel beams. Roberts and other workers were on the tracks nearby, checking reports of faulty switches. TA officials say that just before the accident, Roberts removed the 30-pound cover on the pneumatic switch's control box. Then, the TA charges, Roberts turned an air valve and activated the pneumatic switch, as the A train's last car passed inches away from where he was standing. After the crash, the TA says, Roberts replaced the cover to hide his mistake. But in written reports, the other track workers with Roberts said they never saw the control-box cover removed. After the accident, Roberts, 27, an eight-year employee, was fired from his $44,000-a-year job. Until then, he had a clean work record. Roberts has steadfastly maintained his innocence. Mirrione said he believes that after a hearing, Roberts will be restored to his job at full pay. "If he's guilty, it'll be a shock to me," said Mirrione.