A mysterious track defect somewhere along a major New York City subway line is wearing out the wheels on subway cars, causing a shortage of trains on the lettered lines, the Daily News has learned.

Sometimes it’s a marvel the system functions at all. Hemchandra Singh (left) and Joshua Zailabdeen stacking old subway wheels to be recycled. Gay Burdick tending to an “end destination” sign ...

Subway cars have a tough life ... The trucks are inspected and wheels can be reground to true at the six year mark; they get all new wheels every 12. Once everything is repaired, the shiny ...