The impact of the Belt and Road Initiative (BRI) to global development will be unprecedented and significant, and developmental impact evaluation is therefore central to understanding BRI projects and making informed decisions. Compared with evaluations of individual projects and programs, evaluation of large and mega infrastructure projects under the BRI is particularly challenging and complex in integrating stakeholder objectives, accounting for social benefit and costs, and tracking long-term project impact. In this paper, we summarize the key drawbacks of existing BRI evaluation frameworks, propose a systematic evaluation framework elicitation method based on the inputs from BRI subject matter experts and verified through stakeholder participation, and apply an interim evaluation framework in understanding the Mombasa-Nairobi Standard Gauge Railway project in Kenya, as a proof of concept of a comprehensive evaluation framework. In doing so, we seek to provide a tool for BRI decision makers and stakeholders to assess these projects holistically at planning, construction and operation stages.