Upvote:3
It is somewhat hard to answer this one. You obviously concerned, but not enough to forfeit the course and cancel the trip. You are a bit afraid, but not enough so to overcome you 'flight shame' and hop on a direct plane.
However, you are afraid enough to consider either spending a 25-hour trip on a Flixbus or a 13-hour train trip standing on the corridor. For some reason you are more afraid of Paris than Cologne or Barcelona, and more afraid of trains than buses.
At this point, my honest suggestion is that you do whatever makes you feel most comfortable.
Given that you have already decided to travel, and to travel by either train or bus, the choice between those two matters little.
On a train, you'll be in a carriage with 120 people. A dozen or so in close proximity to you. In a bus you'll be with 60 people. A dozen or so in close proximity to you. There is no evidence that one is significantly riskier than the other.
If you want to minimise your time with people, take a plane or rent a car.