Constrained-transportation directions are described herein. In one or more implementations, navigation assistance is provided for a user that is traveling on constrained transportation. As part of computing the directions, a determination is initially made that the user is traveling on constrained transportation, which may be restricted to traveling along designated routes and stopping at predetermined stops. Based on this determination, parameters indicative of restrictions to the route the user is traveling onboard the constrained transportation are ascertained, such as parameters describing a “line” of a current vehicle, a schedule of the vehicle, and a current location of the user along the vehicle's known route. Using these parameters, rather than merely a location of the user when the directions are requested, directions to a desired location are computed. In this way, users can be presented directions that account for being on a constrained-route vehicle at the time a route is requested.