The young MTA worker who fell to his death this week might have tripped on discarded pieces of track left on the rail bed, a source close to the investigation told The Post on Friday.

St. Clair Richards-Stephens, 23, was working on the upper level track bed at the 125th Street station early Monday morning when he suddenly broke through a wooden railing and plunged headfirst to the lower level. Emergency medical personnel were unable to revive him and declared him dead at the scene.

Transit sources originally said it appeared that Richards-Stephens was “cleared up” in a niche while a work train passed by and leaned back on the railing, which snapped. But investigators now think he might have been moving to clean up debris when he slipped on mud, tripped over the old pieces of rail, and crashed through the wooden railing to the lower floor.

The MTA and the Transport Workers Union Local 100 are conducting a joint investigation.

New York City Transit President Andy Byford said on Wednesday that the agency was looking for all wooden railings in the system and replacing them with stronger fiberglass.

Transport Workers Union Local 100 President Tony Utano on Thursday declined to comment on the new details from the probe, but said the agency must remove the wooden railings.

“This has to be done now,” Utano said. “We don’t want another tragedy to occur.”

This would be the second time in a year that loose rails on a track bed led to tragedy. In June, a train derailed at the 125th Street station in Harlem, injuring dozens and investigators later blamed the incident on a rail that had not been properly secured.