European Rail Traffic Management System (ERTMS) is a standard for the train control and signalling system whose application is spreading throughout Europe. The ETCS (European Train Control Sys-AQ1 tem) level 3 is attracting experts because it is still in the design phase. Many works provide formal models to the verification of ERTMS/ETCS using formal methods, but they did not investigate the validation problem. To deal with this challenge we propose an iterative formal modeldriven approach that helps validating step-by-step a real formal specification of ERTMS/ETCS hybrid level 3. Our approach introduces Domain-Specific Languages (DSLs) to help system experts understand existing specifications that are already proven. To this purpose we extend and apply Meeduse, the only existing language workbench today that allows embedding formal semantics within DSLs. The paper presents this application and discusses the lessons learned from the railway and formal method experts' points of view.