x1 is a public transit system serving x2 and offering services x3 (ka).
This is a zi'evla derived from the lujvo gubma'eci'e, from gubni marce and ciste (public vehicle system). The x2 can be used to specify a geographical region, a target demongraphic, etc. The x3 is used to specify properties enabled to passengers of the system, and can be used with tu'a to very broadly identify stations (tcana) in the network, types of vehicles (subways/trains/light rail trene, buses sorpeka, etc.), and concrete bus lines.