It happened just after 9 a.m. Friday between the Logan Station and the Olney Transportation Center.
Both victims are 17-year-old boys.
Police said the two teens got off at the Olney Transportation Center and walked to Einstein Medical Center.
One boy was stabbed in the left side of the abdomen and is in stable condition.
Police did not say where the 17-year-old boy who is in critical condition was stabbed.
No arrests have been made, but anyone who recognizes the man in the photos is asked to call police at 215-580-8111.