Efficiency is essential to Internet transit trip-planning systems. High-quality data models of transit networks hold the key to efficiency of data querying and network analysis. Because of the schedule control and dynamic nature of the transit network, highway-based generic network models cannot be directly implemented by transit systems. In the process of developing an Internet transit trip-planning system, an object-oriented geographic information system data model, which handles the dynamic nature of transit systems, was developed. In this data model, the entire transit network and its components are modeled as space-time entities (objects) that have start times, end times, and spans of lifetime. A time map object controls the creation and destruction of these objects. Therefore, in the process of network search, only active components of the system (i.e., with transit services available) are built into the network topology. This has greatly reduced data redundancy and enhanced network search efficiency. The data model showed good performance when it was implemented in an online trip-planning service for Waukesha Metro Transit, Wisconsin.