A straightforward, nearly flat piece that nevertheless jumps with movement, Gerson August Leiber’s “Mainline” is simple in concept: just a telephone pole and its lines stretched up and out against a flat white space, hardly worth capturing, with a low horizon portrayed as a jumble of trestle limbs and more telephone poles. Yet the entire composition is alive with energetic, zigzagged lines bouncing around the intersecting components, suggesting connection and communication, and reading almost like a musical score.