We've had many discussions of this over the years, and nothing of substance has changed. Bringing it up now is indistinguishable from trolling, especially prompted by an article about a study of the Alto Tunnel that has nothing at all to do with transit, SMART, or any other kind of rail. On the small chance that you're actually serious, here's the nutshell version.

South of Larkspur (where SMART will end), the r/w is owned by GG Transit or (mostly) cities and the county, where not taken over by adjoining land users. Look at Google satellite and street views for details. Between Larkspur and Detour (where the Sausalito and Tiburon lines split), remnants of the Corte Madera Creek bridge remain, but it's inoperable and unsafe to even be near; the rest is covered by other land uses or has merged with the marsh and its dikes.

South of Detour on the Tiburon line, it's mostly pedestrian and bike path through the shopping centers and housing north of the ridge. Access to the tunnel through the ridge toward Tiburon is long since permanently blocked; the line was abandoned well before the Sausalito line. South of the ridge, from Trestle Glen into Tiburon along the bay shore, it's a bike path where road widening and other development hasn't consumed it.

Between Detour and the Alto Tunnel on the Sausalito line, some is simply gone (parking lots, built over), some is weedy abandoned land (around Baltimore Park), and some is bike/ped path. It was all single-track within living memory, as that was all that was needed for freight after the GGBridge was built and passenger service abandoned, and the unconsumed remaining r/w is mostly only wide enough for that. The Alto Tunnel itself is partly collapsed with no access - and it was never double track, only single - meaning shoring it up for bike use will be muy expensive. South of the Alto Tunnel (through Mill Valley and into Sausalito) it's a bike path except where it's been consumed for streets, parking lots, or built on, and has been for a long time - not available for rail, period.

If you have ideas about resurrecting the other suburban lines, they've been gone even longer and have all been dedicated to other things. Covered with streets, houses, and businesses with perhaps very short identifiable sections if you know what was there and what to look for.

Again, look at Google and historical documents, and dream if you want, but transit south of Larkspur will not be rail. I'm tempted to say forever, but that's a long time. If the Evil BART Conspiracy truly wanted to do it, they would run a line along 101, like the Mouse's scheme perhaps, as the path of least resistance. No construction is starting, no widening for double track is being done, and no r/w condemnation is even remotely planned anywhere south of Larkspur.

If what you're referring to is construction between downtown San Rafael and Larkspur, that's different. It probably will be starting soon though I don't have any good references for when. There will be no tunnel widening - only single track is needed by SMART through the Cal Park tunnel, which was reconstructed several years ago to be shared between a bike path (existing) and SMART. There will be no condemnation - SMART already has all the r/w it needs.

OK?