Environmental sustainability is an increasingly important subject in the railway sector. Literature has revealed that transportation companies target multiple sustainability-related improvement areas and follow different strategies to improve in these. For railway operators, train modernization provides key opportunities for technical, financial, and environmental improvements halfway through the lifecycle. Using design science research, a framework was developed that is aimed at integrating sustainable design principles and sustainability-focused requirements in train modernization. The framework was tested and demonstrated by means of application to the train modernization project of an intercity train at the Nederlandse Spoorwegen in the Netherlands. The results reveal three key design mechanisms for sustainable modernization frameworks. Firstly, sustainability should be considered as early as possible in the design process. Secondly, such frameworks require efforts to simplify the way sustainability principles and impacts are considered, being careful not to oversimplify. Thirdly, the use of sustainability-focused requirements and budgets facilitate the process of prioritizing design decisions in relation to the effects they have on various environmental impacts. The design process of the framework not only provides a better understanding of the challenge of integrating sustainability, it also presents a generalizable approach that can be adapted by other organizations in the transportation sector.