Gables and Vista Marin residents within 100 feet of the SMART tracks should anticipate sound levels of 80dBA; well above the standard background level of 48dBA.
SMART conducted a noise analysis as part of its Environmental Impact Report in 2005. The 2005 Draft EIR can be found here:
http://www2.sonomamarintrain.org/index.php/docs/eir/#DEIR (see Section 3.7 and 3-132 for noise levels for a Diesel Multiple Unit – or DMU.)
The Final 2006 EIR can be found here:
http://www2.sonomamarintrain.org/index.php/docs/eir/#FEIR (See page 4-52 to see any revisions/corrections)
No current background noise level measurements were taken at Vista Marin or the Gables, the closest is Smith Ranch Road at Gallinas Creek where the levels were 48 dBA.
Based on DMU projections homes within 100 feet should experience 78.8 dBA; and homes within 50 feet 88 dBA (train accelerating in notch 5 at 50mph – presumably the train will be accelerating north out of Civic Center Station). This federal railroad administration document explains dbA.