It would be interesting to see how BART's aluminum beer can cars would fare in a collision. Their construction is so sub-standard they've crinkled in normal use. A fairer example of a heavy rail transit car would be an NYC subway car.

There are several obvious factors that affect the weight of a light rail car. Articulation is clearly the most significant as it adds the weight of an extra truck, even if it is not motored. But the advantage is versatility - it can negotiate a greater than 90 degree curve such as the one at 30th and San Jose in San Francisco.

Being able to run in the street or cross roads at grade means you have to build in structural strength to cope with inevitable collisions. Generally this would translate to more steel, meaning more weight but more strength and durability. Also a lot easier to repair.

BART's broad gauge and third rail makes it totally incompatible with the existing NWP. The North Bay neither needs nor can afford a totally new grade separated rail system that would cost as much as high speed rail.

That much capacity is complete overkill. Consider the new Seattle system. Seattle is a major metropolitan area and one of the richest in the country. They have determined that light rail is adequate to meet their ridership needs, which is clearly greater than lightly populated Sonoma County. Also may you actually have to live in Sonoma County to understand that this ia still a semi-rural area with a stubbornly low level of public transit use.

Another important factor is availabilty and one that I suspect has affected Seattle's choice of mode and that of other cities that are opting for light rail. Suppliers are proliferating, which means realistic costs and more innovation. You might in time even see some real mass production, impossible with BART's unique stuff.

Light rail makes a humble starter system possible. Petaluma to Santa Rosa would be cheap by comparison to Marin County. In Marin I believe you will have to elevate in downtown San Rafael and I think the line from the outset should go all the way to Marin City along the 101 corridor. Having some operations up and running in Sonoma would be very helpful in convincing Marin voters to shell out the extra bucks needed in their more densely developed area.