Thanks James for looking into this. Burr Wilson covered the history of the line well. The Sausalito-CAzadero was separate railroad from the Tiburon-SAn Rafael and points north line until 1912 or something. But thereafter you had the joint ownership which could account for some of these concrete posts. So I agree you could be on to something. Also, one of the concrete posts is 7 over A and theis a 7 over A post on the electric interurban in Sausalito seen in oldtime pictures.

If the North Shore used concrete for their posts, then I think your explanation would explain that particular post.

Interesting issues!