The signal Adam talks about was the Distant Signal on the Approach to the Automatic Interlocking where the PE, later SP Line to Long Beach crossed the UP. In my old age, I can't think of what the Station Location name was, but in the days of the PE Passenger Service, this location was controlled from Dominguez Junction Tower (info. from James A. (Jim) Mahan, retired SP Towerman). When the Tower was closed and shut down, this Interlocking became an Automatic one. You could see it from the Long Beach Freeway and the LA Metro Blue Line crosses there now on a Bridge. As far as official ABS on the current day San Pedro Subdivision, per UP LA Area Timetable #4 that is current edition, states that, "ABS between MP 3.4 and MP 4.2." This would be between Riddle and what UP refers to as "The Federal". However in the days before the Alameda Corridor was put into service and Hobart Tower was closed and shut down, the ABS really extended from the Interlocking Limits at Hobart Tower to Automatic Interlocking at MP 5 whre the PE, later SP, now UP crosses, including the Automatic Interlocking where the LAJ RY crossed at MP 3.6 (this info. per Ed Wolf, retired UP Maintainer and railfan).