This article presents findings from a qualitative in-depth analysis of a four-year Swedish national policy initiative where six public agencies were commissioned to produce a strategic plan for a transition towards a fossil-free transport sector. The aim of the article is to provide empirically grounded insights on principles and practices of importance for building transformative capacity in strategic, long-term transport planning. In the analysis, the concepts stewarding, unlocking, transforming and orchestrating are applied to explore and discuss transformative features of the policy initiative. Altogether, several elements of transformative capacity were developed through the process. Of specific importance was the establishment of an open and explorative approach to carrying out the commission, and ways in which the organizations involved started to challenge dominant perspectives and analytical practices in conventional transport planning. Shared principles and practices for analysis and assessment were developed, which allowed for a broadened consideration of climate mitigation measures. However, due to a lack of coordination with formalized planning settings and a lack of political decisions to sustain the commission, there are yet no signs of the work influencing conventional transport planning. Even so, gained experience and insights from this case can inform future change-oriented initiatives.