GREATER BOSTON has one of the most expansive transit services for the disabled in the nation — and it shows in the bottom line. The Ride, the MBTA’s fleet of white-and-yellow cars and vans, is a lifeline for many residents who would otherwise be stranded. But it’s become a financial disaster. The Ride’s costs have quadrupled in a decade, and show no signs of abating. Each trip costs $40. The system carries less than 1 percent of the T’s passengers, but consumes almost 10 percent of its budget. It’s a vital, necessary service, but not sustainable in its current form.