In this paper an online optimisation approach is proposed which can be used to find an appropriate combined schedule and container assignment in a Network Design Problem under uncertainty. For this, a simulation based approach on a multi-period time window is proposed, moving forward on the time window after each decision made, assuming that the status of the system is updated as soon as the stochastic and unknown elements become deterministic and known. This approach provides new insight and knowledge into synchromodal and multimodal planning problems. The results of the approach are compared to the results of three simpler online optimisation methods and to the solution of the offline approach where all information is known.