“…The expected speedholding operation is not perfectly realized; instead, it is approximated by a mixture of coast-power pairs and speedholding. It might be because that the speedholding corresponds to the singular arc of the train control problem given in Section 2 (Albrecht et al, 2015b;Howlett, 2000;Khmelnitsky, 2000;Liu and Golovitcher, 2003), and it has been known that the singular arc may not be perfectly estimated by the so-called direct method including RPM for solving the optimal control problems (Betts, 2010;Garg, 2011;Patterson and Rao, 2014;Rao et al, 2010). To obtain a high-accuracy speedholding operation by RPM, one may explicitly provide the singular arc conditions as path constraints (Betts, 2010;Patterson and Rao, 2014) in the original optimal control formulation.…”