A Possibility

Orinda CA was served, briefly by a small narrow gauge around the turn of the 20th century; The California & Nevada RR. The terrain in the photos looks like it could be in that vicinity. A roster for the line when it opened lists two Porter locomotives, two combines, one boxcar and four flat cars. The locomotive in the pic has the general outline of a Porter product, although it's impossible to tell for sure. The four "gons" look like they could be jury-rigged from flat cars, possibly during construction as the track appears to be recently laid. The wood-burning stack, oil headlights and general texture of the photos are all consistant with the 1890's era when the C&N was built.
A few items of interest.......the locomotive has a forney-type tender with an additional saddle tank - somewhat unusual although certainly not unheard-of. Link & pin couplers, and the dangling devices between the cars look more like safety chains than air lines. That and the brakemen riding the cars suggests no train air, although there is some kind of device under the center of the cars which could be an Eames vacuum brake canister.
The California & Nevada was a small, short-lived narrow gauge; mostly a passenger hauler with a small traffic in agricultural products. No mention in my sources of mining activity on-line. If your source was correct in that the location is in the vicinity of Orinda, then the C&N during construction is a likely guess.
J