For the record I beleive that (BART) Bay Area Rapid Transit is 5'6" gauge. Three locations BART had and in some cases still does have dual gauge interchange trackage. Fifth Ave Oakland Maintenance Facility was build with dual gauge interchange connection with SP and WP (Retired). Hayward Maint. Faciity also had two dual gauge interchange connections one for SP (Now UP and still in use) and one for WP (Retired) on the East and Wast sides of the yard respectively.
Concord also had a dual gauge interchange with the Sacramento Northern Ry. The one remaining dual gauge intechange is used for deliver of welded rail which is transferred from a standard gauge rail train to BART's wide gauge rail train. Also Roaring Camp and Big Trees NG RR had a dual gauge interchange with SP at Felton.