Some recent literature reviews on TNDPs can be found in Desaulniers and Hickman (2007), Guihaire and Hao (2008) and
Kepaptsoglou and Karlaftis (2009); a less recent review was reported in Chua (1984). As proposed by Guihaire and Hao
(2008) several problems can be identified and classified on the basis of the decision variables considered. Assuming as design
variables (a) line routes, (b) frequencies and (c) timetables, they identified three main problems resulting from consideration
of only one kind of decision variable in the problem, and three other problems that jointly consider variables (a) and (b),
variables (b) and (c), and variables (a–c). The survey proposed by Guihaire and Hao (2008) examined contributions according
to this classification.