fugetaboutit

BART's third rail requires full grade separation from surface roads.

There are at least seventeen grade crossings on the Dumbarton line to build over- or underpasses between Redwood City and the BART line in Fremont.
Each grade separation costs at least $150M, depending on size and land acquisition costs.
Recent examples: Caltrain's grade separation at 25th Ave in San Mateo cost $180M.
San Bruno grade separation cost $155M.

The Dumbarton line grade crossings are:
Redwood City: Second Ave and Fifth Ave
Menlo Park: Chilco St, Willow Rd and University Ave
Newark: Willow St, Spruce St, Ash St, Filbert St, Sycamore St, Cherry St, and Cedar Blvd,
Fremont: Blacow Rd, Dusterberry Way, Maple St, Fremont Blvd and Paseo Padre Pkwy

At an average cost of $150M, you're looking at $2.55 billion, just to build the seventeen overpasses. Add the cost of two new bridges, rehabbing the roadbed & rails, rebuilding the trestle on the western side near University Ave, and a new signal system.

And then there is the issue of dispatching trains on dual-gauge tracks. Would BART be in control? Or Caltrain?

Don't hold your breath for dual-gauge on the Dumbarton line. It won't happen.