If I understand correctly this relates to the number of departures - so the number of buses, trains, trams, etc. that run from one end to the other of a line. The Finnish version of the email uses "lähtöä" in the same context which supports this.
It's a bit of nerd jargon (some of my best friends are transport planners!) but if they meant "person getting on a vehicle" I'd expect them to use 'boardings', and if they meant "people taking a trip from one place to another" I'd expect them to use 'journeys'.