Buzz From United States of America, joined Nov 1999, 697 posts, RR: 24 Reply 2, posted (7 years 1 month 6 days 7 hours ago) and read 3747 times:
Hi KETA, Buzz here. Those are "rain gutters" to channel rivulets of rain away from the cabin doors as the rain rolls down from the top of the fuselage.
It seemed that the DC-10's "rain gutters" didn't work as well as the Boeing 727 / 737... at least in "sunny" Portland Oregon (it tends to rain here). I'd joke that the DC-10 was designed in southern California... sunny warm weather.
Zkpilot From New Zealand, joined Mar 2006, 4739 posts, RR: 10 Reply 3, posted (7 years 1 month 6 days 7 hours ago) and read 3741 times:
They are there to stop rain dripping onto passengers or onto the floor of the aircraft when the door is open for boarding/deboarding.
They are aligned so as to be as little drag penalty as possible... since most aircraft fly with a slight nose up attitude, the rails are angled downwards at the front so inflight they are almost level.