Blocking constitutes an important rail freight transport operation, by which cars with potentially different origins and destinations are grouped to be moved and handled as a single unit, yielding economies of scale. We address the tactical block-planning problem for intermodal railroads, which has been little studied so far. We propose a new block service network design model explicitly considering the specificity of intermodal rail, and which may be solved using commercial software for realistic sizes. We illustrate the performance and interest of the proposed method through an extensive case study of a major North American railroad.