We analyze the effect of unbalanced demand on the type of optimal service structures, e.g. trunk–feeder or direct lines, on a simplified linear urban corridor with three zones, two origin–destination pairs and a common link, where demands can be served with up to two complementary, substitute or exclusive lines. The best lines structure is identified as a function of the demand imbalance under total cost minimization (operators plus users). The conditions under which services with transfers dominate are identified and discussed.