There is kind of an official explanation for the non-standard gauge: Western Railroader June 1965 Vol 2806 Issue 305: “Clara (the test car) arrived at Concord

There is kind of an official explanation for the non-standard gauge: Western Railroader June 1965 Vol 2806 Issue 305: “Clara (the test car) arrived at Concord from Sacramento in a special Sacramento Northern train. She rode on a WP flat car instead of on the rails since she has a 5′ 6″ gauge, nearly a foot wide thant the 4″ 8.5″ standard gaguge used by most existing transit systems and all the nations railroads. Parsons Brinckerhoff- Tudor Bechtel, the districts consulting engineers, said that exhaustive studies show the wide gauge provides great stability and smoother riding qualities for the rapid transit trains.” [0] However, the most likely explanation is that during planning, dubious reasons like the one above were given to convince officials that a non-standard gauge would be better. After it was built, the company which build the first rolling stock had a monopoly. So the reason most likely is classic lobbying. [0] http://www.bayrailalliance.org/question/why-does-bart-use-wider-non-standard-guage-rails/ http://www.bayrailalliance.org/question/why-does-bart-use-wi...