PLAINVILLE — Earth-moving machines and hard-hat workers crawl over the grounds of Plainridge Racecourse, busily building a 1,000-car parking garage that the flagging racing industry could not possibly fill.

For the partnership that owns the racetrack, the $20.5 million garage is a featured element in their application for the state’s sole slot parlor license. The harness racing track, at Route 1 and Interstate 495 near the Rhode Island border, will make a case that its development proposal is the furthest along — in planning, design, permitting, and construction — and would be the quickest project to open and to produce tax revenue if awarded the license.