Bonds sold by the Sonoma-Marin commute rail agency are rated by financial analysts as stable investments, but still are graded below other Bay Area rail systems because the trains are not yet running.

Fitch on Monday rated the Sonoma-Marin Area Rail Transit District at A and Standard and Poor's on Tuesday gave SMART a rating of AA.

In comparison, Fitch gave AA ratings to the Bay Area Rapid Transit District (BART) and to the Santa Clara Valley Transit Authority.

"They are more established systems," said Scott Monroe, an analyst for Fitch in San Francisco. "Since SMART is new, and has yet to construct the system, SMART took a conservative approach to cost, tax revenues, ridership and farebox revenues," Monroe said.

However, Fitch is also taking into consideration what it considers some negatives, such as a low percentage of ridership compared to the number of workers in Sonoma and Marin counties and a low farebox recovery ratio.

SMART told analysts that it expects to carry 3,000 riders a day and that farebox revenue is forecast to be 22.9 percent of operating costs.

In San Diego, the Coaster rail system had a farebox recovery of 39.6 percent, but the Sprinter had 21 percent for the six months ending in January. The Santa Clara Valley Transit Authority had a farebox recovery rate of 15.7 percent in 2011.

SMART is developing a fare schedule that is based on the length of the trip. In it's 2009 strategic plan, SMART estimated that the average one-way fare would be $4.50.

SMART General Manager Farhad Mansourian said forecasts provided to analysts are purposefully conservative and have been reviewed by the Metropolitan Transportation Commission.

Standard and Poor's analysts give SMART bonds a AA rating, based on the strength of the quarter-cent sales tax that will be used to repay the bonds.

"The stable outlook reflects our opinion of the strong and diverse tax base and good coverage of annual debt service with projected long-term growth in sales tax," according to a summary by the rating service.

Standard and Poor's rated both BART and the Santa Clara authority at a higher level, AAA.

Standard and Poor's notes that sales tax revenues grew strongly between 1990 and 2007, fell 19.2 percent from 2007 to 2009, but increased 11.6 percent in 2010-2011 and are up for the first three quarters of 2011-2012.

SMART is scheduled to convert $190 million in construction bonds the week of April 16 to fixed rates that are expected to be between 4 and 5 percent, Mansourian said.

The bonds were previously sold at a variable interest rate of 1 percent and the funds that were raised, $171 million, put into escrow because of the threat of a ballot measure seeking to overturn SMART's sales tax, the major source of funding.

Those funds were released from escrow after SMART opponents, RepealSMART, were unable to gather enough signatures to put a measure on the November ballot in an attempt to overturn the SMART sales tax.

The $171 million is almost half the cost, $366 million, of building the initial segment of the line, 38.5 miles from Guerneville Road in Santa Rosa to downtown San Rafael.

Service is scheduled to begin in late 2015 or early 2016.