Digital infrastructures enable delivery of information services in functional areas such as health, payment, and transportation by providing a sociotechnical foundation for partnership governance, resource reuse, and system integration. To effectively serve emerging possibilities and changing purposes, however, a key question concerns how an infrastructure can be extended to cater for future services in its functional area. In this paper, we approach such digital infrastructure growth as a challenge of aligning new partners whose digital capabilities spur innovative services that attract more users. We advance an initial typology that covers four growth tactics (i.e., adding services, inventing processes, opening identifiers, and providing interfaces) with the potential to set extension of infrastructures in motion. We then explore the proposed typology by investigating the ways in which its particular tactics successfully extended the scope of a digital infrastructure for public transportation in Stockholm, Sweden. Our insights invite IS scholars to engage more deeply in the development of growth tactics that achieve infrastructure extensions necessary for improving the durability of service delivery.