Dellner is a company with many years of experience in the rail industry and provide train connection systems. The headquarters is located in Vika, Sweden where the focus is on couplers engineering, assembly and aftermarket service. A train coupler consists of several subsystems and this investigation will focus on an external rotation lock located below the subsystem center section. The external rotation lock consists of a guide rail locking the rotation. However, there are some issues with the guide rail.This investigation will study the external rotation lock and apply a concept development process, based on theory by Ulrich & Eppinger (2012), in order to develop new conceptual designs for the current external rotation lock, given the issues.The results include six 3D-designed concepts which are described and then compared via a concept screening which Dellner can decide whether to continue with the highest scored concept or further improve, revise and combine the concepts before selecting a single concept to make a detailed design and set the final target values.