@Alexander7,
Transition overground means in that instance to travel via or over the rail. i guess its over because of the picture. you are transitioning ie: transiting from one place to another, and youre doing it by a method which travels along the ground rather than in the sky or over water or lava or a pit of snakes or something. i think
oh actually i like the last answer, as in as opposed to underground. that makes much sense.