This article highlights an application in the area of decision support for planning transports in a railway company utilising constraint programming and a flexible design which has been sucessfully tested on real world data. We discuss the problem formulation for the co-ordination of distinct subproblems, the allocation of track resources to transports, the allocation of vehicles to transports, and the allocation of personnel to perform the transportation tasks in a railway company and the development of heterogenous constraint model which is usable also for other production planning problems. Using constraints as the key technology, we discuss approaches to find interfacing principles to combine several solvers.