Inspired from the digital internet, the Physical Internet (PI) is defined as a global standardized and interconnected logistics system. This paper studies the Road-Rail assignment problem which consists on assigning inbound trucks to the PI-docks and assigning the unloaded PI-containers to the train's wagons. A mixed integer linear programming mathematical model (MILP) is proposed with the objective to minimize both the number of used wagons and the internal travel distance by the PI-containers from the PI-docks to the wagons. Moreover, a heuristic based on the first fit algorithm is proposed for generating an initial grouping of containers and a tabu search meta-heuristic is used after to find better solutions. Finally, the proposed methods are tested on several instances and the obtained results are presented.