In the current climate (pun intended), traveling by train is the most ecological alternative.
Austrian Railways have understood this and are offering sleeper trains from Germany to Austria. Leaving Cologne in the evening, you arrive at Vienna central station early in the morning, with a full day ahead for meetings or sightseeing.
Swedish Railways have just announced plans for an overnight Stockholm to Brussels train, which would add a much needed East-West connection.
Alas, Deutsche Bundesbahn seems to ignore the opportunity.
(NJ 40421 to Vienna, 8/1/20)