External transport costs are becoming an increasingly important factor in the choice of transport routes. According to sustainability principles, the first choice criterion should be the route with the least harmful impact on the environment and health. Sometimes the criteria for transportation costs are in conflict with environmental ones. There is a need to optimize them to maintain the trend of sustainable transport development. Cost and external cost behaviors of intermodal maritime–rail container transport from China over different European ports to central European destinations were examined. The aim was to determine the differences in dependency on transport route length of these two variables that are able to partly explain their different features. The complete functional dependency of external cost in maritime transport and transport cost in rail transport on route length is determined. External cost dependence on the transport route length in the railway segment was strong but incomplete. Maritime external cost share in the total external cost of combined transport was at least 83%. The weak negative dependence of maritime transport cost on route length probably reflects efforts to maintain the competitiveness of consolidated longer routes. This article contributes to the ongoing discussion of seaport competitiveness within Europe.