The tram has just become stationary at the stop. What is the right course of action?

Your answer is fully correct. When a tram has just stopped at the stop, according to German traffic rules, you must ensure that the tram is not endangered and not impeded. This means you have to give way to passengers boarding or alighting and avoid any actions that could interfere with the tram's operation. Using the horn (warning) in this situation is not permitted, as it could startle pedestrians or passengers near the tram. So, always prioritize safety and smooth operation by not endangering or blocking the tram. This rule helps protect passengers and ensures public transport runs efficiently.