PHILADELPHIA (WPVI) -- Philadelphia police said a 41-year-old man was shot in the head and killed while riding the Market-Frankford line early Monday morning.Police said it happened around 12:50 a.m. on the westbound elevated train near the station at Kensington and Allegheny avenues in the city's Kensington section.Officers were searching for four suspects who left the train. Investigators said it appears to be a robbery.Police are currently working with SEPTA Transit Police to review surveillance video.The victim was taken to the hospital where he was subsequently pronounced dead.