SMUD (like all municipal utilities) cannot sell power outside its boundaries. It can buy power (and does), and it's transmitted (unless coming from SMUD-owned generation like the Upper American River Project) over PG&E lines. Yes, under the right circumstances PG&E could cut off SMUD and force them to use only their own generation (or other generation within its own territory - there are several plants), which in the short term could cause SMUD to use rolling blackouts to manage any shortages that develop.

Using railroad r/w for power lines is an ancient and honorable practice, done in many places Back East. If they were feasible, PG&E would build them. However, overhead power lines are a NIMBY item in California (and elsewhere) so it's unlikely that it would be done. And they would need to connect with major substations, which aren't present near most of the NWP/SMART lines.

Of course PG&E could cut off BART power, but most likely in spots rather than the whole thing at once. BART uses commercial power not its own, so if for any reason power is cut off on a circuit that feeds a BART substation BART loses power. If it's like most electric transit systems, BART can continue operating, at reduced speed etc., when one substation is down, but two or more adjacent substations going down can bring trains to a halt requiring a bus bridge. I don't know if PG&E has separate HV circuits feeding BART, not interconnected with the towns along the line, but wouldn't be surprised if it does.