A subway-track supervisor was electrocuted yesterday after he slipped in the rain and fell onto a live third rail at an elevated A-line station in Queens.

Married dad James Knell, 45, died at 4:30 a.m. after he and a crew had just finished a rehab project replacing outdoor track panels at the Beach 90th Street-Holland station in Rockaway Beach, authorities said.

“The third rail was turned off during the work,” said John Samuelson, TWU Local 100 president.

“It was then energized at the end of the job to run a test train. Jimmy then went back on the tracks to retrieve a bucket. It was wet, and he slipped and fell onto the third rail.”