Service resumed early Wednesday on a critical New York commuter rail line, the morning after a massive fire broke out under Metro-North tracks in East Harlem. Hundreds of commuters were left stranded Tuesday night at New York’s Grand Central Station, after the fire erupted around 6:45 p.m. Early Wednesday, trains jammed with commuters were moving through the fire area for the Harlem, Hudson, and New Haven lines, and the agency said all would operate on a Saturday schedule until repairs were completed Friday. The blaze appeared to have been sparked by a tractor-trailor, and the city’s fire chief said explosions rocked the area, with heat so intense it blew bolts out of steel. The cause is under investigation.