When adding a new train to an existing timetable, this often has to be done in a way that gives an efficient train path for the added train, while the existing trains in the timetable only can be shifted in restricted ways. A first step in the addition procedure is to find all the available track capacity that can be used for the train to be added, given its earliest possible departure time and latest possible arrival time. This available track capacity will form one or more capacity corridor from theorigin to the destination. In this paper we propose a method for handing all available capacity corridors that can be used when adding the train. The basis of the method is an optimization model, which includes all scheduling restrictions, securing that the capacity corridors are both feasible from a security viewpoint and also feasible with respect to the already planned trains.