What is the right course of action here?

You selected the correct answer: you must not overtake the moped before the rail crossing. In Germany, it is forbidden to overtake at or just before level crossings (rail crossings) without barriers or with half barriers, because of the increased danger. This rule ensures that vehicles do not get trapped or stuck on the tracks while overtaking. Even if the way appears clear or you want to overtake slower two-wheelers, you must wait until you have fully crossed the railway tracks before overtaking. This safety rule is strict to prevent accidents at crossings.