>It is my understanding that the Niles/Tracy line was the primary freight route from the Peninsula, as well as San Jose, and points south to Sacramento and Roseville from the completion of the cutoff in 1910 until the completion of the Suisun Bay Bridge in 1930.

Tracy-Redwood was the primary route for Rsv-SF freight well after the Bridge was built because it wasn't possible to move from the Cal-P to the Niles line at West Oakland and vice versa until the connection was built where the BARTD overpass is now. This occurred several years after Oakland Pier was abandoned in 1958 (the connection was built around 1961).

Nor was it possible to go from Sac via the Bridge to Tracy without a lot of very inconvenient movements at Martinez and union agreement red tape. The proposed but never built connection to the Mococo line for using the San Ramon branch to get to the Tracy line was intended to bypass the traffic jam named Oakland.

The primary route for tings like the perishable traffic from Salinas/Wastonville was Wastonville Jct-San Jose-Niles/Newark-Altamont-Tracy-Lathrop-Brighton-Elvas-Rsv for the same reason (and the same for traffic going the opposite direction). Installing the connection at the BARTD overpass is what slowly killed the Altamont line between E Pleasnton and Midway.