The worker apparently got into a dispute with a 47-year-old man sleeping on a bench.
It happened on a southbound D and R trains platform at the Atlantic Avenue-Barclays Center station at around 1:45 a.m.
The 62-year-old MTA worker was cleaning the area with bleach when the incident occurred.
At some point, the two began arguing after the man objected the worker's cleaning efforts.
The dispute escalated with worker slashing the man above the left eye.
He was treated for injuries at New York-Presbyterian Brooklyn Methodist Hospital.
Charges for the MTA are pending.
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