<p>The Klang Valley Mass Rapid Transit (KVMRT) rail system is a central part of the development of the Kuala Lumpur public transportation network with MRTC the project owner and MMC Gamuda KVMRT (PDP SSP) the Project Delivery Partner. The first MRT line stretches from Sungai Buloh to Kajang (SBK) with a length of 51km, while the second line from Sungai Buloh to Serdang to Putrajaya (SSP) has a length of 52km. The majority of both lines is supported on viaducts which weave their way through the urban environment. This paper presents the development of the viaduct design for the SBK Line and the lessons learnt that were incorporated into the SSP Line viaduct design. The process of optimising the design to meet the requirements for both the SBK and SSP Lines will be discussed while some key technical aspects of the viaduct design will be highlighted.</p>