The TNDP has been extensively studied from different angles, considering different decision variables, different solution
approaches, different objective functions and different assumptions on demand elasticity; for a thorough examination of the
general problem the reader should refer to Ceder and Wilson (1986). It is an extremely complex and difficult non-convex
problem (Newell, 1979; Magnanti and Wong, 1984; Baaj and Mahmassani, 1991) that is usually formulated as a mixed
non-linear optimisation problem, where some constraints are non-linear and others are not expressible in a closed form
(transit assignment constraints).