In the initial release, all tracks will have standard gauge. There will however be multiple track types (with different rail profiles, sleepers/ties etc.) that determine the maximum track speed and axleload. The availability of the types will change over time, e.g. high-speed tracks won’t be available in 1850.
When building tracks, the desired maximum speed can be set as a design parameter, so that the player can be warned if the curvature is too high. There will also be an appropriate data layer to reveal maximum speed and other track properties.