In one implementation of a modular rack system, a rack module (22, 24, 222, 522, 722, 724) comprises a bay (30, 730) comprising a first side wall (32, 732), a second side wall (32, 732) and floor (36, 736) and an intermediate wall positioning mechanism (280, 380, 390, 392, 790, 792) to support a wall (282, 782) at different spacings with respect to the first side wall (32, 732). In another implementation of the modular rack system, a utility bay (148) extends across rack modules (22, 24, 222, 522, 722, 724).