SB mile posts

I apologize if this has already been discussed elsewhere. I looked through the 7 pages of the forum, but could not find any info. Basically, I would like to know what the "SB" refers to on mile post markers located on Pan Am's Freight Main Line between Andover and Atkinson. The SB markings appear on several concrete mile post markers. The SB and original mileages have been painted over and replaced with the current mileages. I have heard everything from "South Boston", "South Berwick", "Salisbury Beach", etc. I was unable to locate any info in previous timetables, maps, etc. Any info would be greatly appreciated. Thank you.

I believe that SB is South Berwick, the point where the B&M - and the Eastern - ended in the 19th century, and shared the Portland, Saco and Portsmouth route to Portland, until the B&M built its own route.

PBM

Thank you very much. I appreciate the prompt response. We had always suspected it was South Berwick given the mileages embedded in the posts. Thought it better to ask than assume. Nice to see things from that era are still being used.