The paper deals with the problem of transit system design for a mixed fleet of electric and internal combustion buses and introduces a model for the vehicle type choice that involves computation of lifetime internal and external cost. Unlike previous works focused on transit network design problem, this model assumes the set of routes as fixed. It introduces instead different fast charging alternatives and constraints related to battery autonomy, energy consumption and power transfer from the electricity grid. Results of a real-size numerical application carried out on a transport corridor in Rome are illustrated.