John Kolvenbach's Fabuloso, currently closing out Third Rail Repertory's first season at the World Trade Center, is a rompy little domestic fable depicting the lifestyle clash of two very different couples. Like an inversion of Who's Afraid of Virginia Woolf?—complete with rapacious boozing and nonsense nursery rhymes—Fabuloso posits not that conventional relationships are fundamentally hollow, but that they're fundamentally unfun. There's much to recommend about this production—why, then, does it rank among one of this fine company's weakest offerings? Momentum has a lot to do with it—Fabuloso benefits not a bit from leisurely interludes between scenes. It doesn't help that Kolvenbach's script, while heavy on silliness, scrimps on the emotional ballast. Given a goofball script and direction that stalls any sense of urgency, the good-hearted Fabuloso is less rewarding than the Third Rail's customarily sophisticated productions.