This paper studies how transit lines should be developed in polycentric cities. In several growing metropolitan areas, local authorities have to decide whether to rely on existing radial lines connecting suburban areas to the city-center, or to develop new circular lines directly connecting suburban areas. An efficient transit system aims at reducing external costs of transport (congestion and pollution) by attracting private car users. We study the effect of two types of policies on the modal split. First, we compare the effect of three administration regimes (public, semi-public and private) on the external costs of transport. Second, we consider the opening of a new transit line directly linking the suburbs. We find that it reduces aggregate user cost but is not Pareto-improving unless crowding is high on existing transit lines. Our analysis is complemented by a numerical illustration based on an open source Fortran program. This tool needs a relatively small set of data and can be used by policy makers wishing to investigate the pricing reforms or the possibility of opening a new transit line for a specific case study.