In looking through LIRR H10s photos I found several with passenger steam heating lines on pilots and tenders. The specific photo you posted above was featured in Ron Ziels' The Pennsy Era On Long Island . The caption dates the photo to March 1949 and was taken at Montauk. Apparently due to a hostler screw-up assigned K4s 518 had to have its fire dumped and local freight H10 109 was pressed into service to pull the South Shore Express west. With a good sized roster of G5's and leased PRR K4s. G5's, and E6s' I doubt the LIRR would assign H10's to regular passenger service, but fitted with steam lines they could be pressed into service when exigencies arose.
The first car in the train is a PRR PBM70 - a rather rare bird. There are quite a few photos showing PRR P70's on the LIRR - usually on non-commuter trains like The Cannon Ball, the Shinnecock Express, and The South Shore Express. The Cannon Ball featured heavyweight parlor cars for well healed patrons heading out to the Hamptons for the weekend.
Ed Rappe