Every now and then, as yet another peppy cliché prances across the stage of the Circle in the Square Theater, you may pause to ponder the pioneering achievement of 'In Transit,' the singing portrait of New York City subway travelers, which opened on Sunday night. After all, what you're listening to often gleams with the blended polyphony of a good-size band. Yet not an instrument has been used in the performance of this a cappella musical, staged by the Tony-winning director and choreographer Kathleen Marshall. Everything we hear, as we are told in a (sung) preshow announcement, is created by human voices. Acknowledging this is rather like admiring the ingenuity that must have gone into a sentimental picture of a rainbow, perhaps with the Care Bears in the foreground, rendered entirely in bottle caps. It's definitely something to have achieved such visual specificity out of bottle caps. But it's still a picture of a rainbow with Care Bears.