The first BART car off of the Rohr assembly line was A car #101, one of ten prototype cars designed to resolve planned and unplanned “bugs” with the BART cars. The 101 was delivered in August 1970, and was the only A car rolling around until mid-November, thus the picture of this short “one car train.” Also note the silver painted cab – the prototype cars had an odd variety of “liveries.”
To note, at the time there were three boxy “Laboratory Cars” which also operated up and down the “[Southern] Alameda line.” They were originally built for operation on the Diablo Test Track located in Concord.
At the conclusion of the prototype car testing program, most of the “dented and battered” prototype cars were replaced with production cars of the same number. The original 101 was scrapped and the new replacement 101 rolled for a few years until becoming B car 821, thence B2 car 1821 (and scrapped).
"The Two Bagger" is meant to be a place to store more "blog" style posts on various cars, pictures, and random tidbits. At BART, a "two bagger" is a rather informal name for a two car train. Two car trains rolled in revenue service back in 1972.