I found this video at TruStack's channel on YouTube. TruStack reports:
Before there was BART, before there was AC Transit, there was the Key System, a series of train/bus routes in the east bay and SF. Of, GM bought them up and shut them down.
This is a great video. About 20 minutes long. It was made in 1945 and contains a kind of history of the development of transportation in the East Bay. It discusses the "pre-bridge" era of transport, then the opening of The Bay Bridge and how it had a level for trains.
The video has great scenes of trains zipping through Oakland and Berkeley low hills areas. It also has scenes of downtown Oakland in that day. You can recognize the 18th and Broadway intersection, for example. It's worth taking time to view the video.