The paper proposes a continuum approximation-based optimization model, considering vehicle emissions, to jointly optimize a bimodal transit network service in a heterogeneous environment where the passenger demand is not uniformly distributed over space and time. Correspondingly, the designed transit service characteristics, including the spacing of lines and stations, line headways and lengths, may vary over space and time to better cater for variable passenger demands. A successive substitution solution approach and an endpoint method are employed to generate an optimal solution. Experiments are conducted to illustrate the properties of the model and validate the solution methods. The results indicate that a trunk-feeder bimodal transit network is preferable under a higher-level heterogeneous demand. A rail-bus system is much more desirable compared to a bus rapid transit (BRT)-bus bimodal system in a larger and more developed city. Another interesting finding is that the scale of at-stop emission cost reduction is significantly larger than that of inter-stop emission. Compared to the conventional bimodal transit network design models without accounting for vehicle emissions, the incorporation of an emission factor into the optimization model can reduce both the total system and emission costs, which consequently achieves a more sustainable bimodal transit system.