I did not get the impression there was any caution shown about watching and waiting, but instead a few spoke of using masks, open windows, and having your own sleeper car...as if that is a guarantee at this time. I don't think it is.
Night trains are making a comeback in some parts of Europe like Austria, Germany, Switzerland, France, Nordic countries to name a few, and in other parts they've never gone out of existence like in Italy apparently and no doubt in other countries. The reasons for this comeback are manifold. Right now, there is an increased interest in train travel including night trains because it's summer and the holiday season (some night trains operate only during the summer season) and because Covid causes changes of behaviour patterns and holiday patterns. Train operators report that offers for private compartments or private cabins in night trains find increased interest. And since trains are currently far from fully booked, train operators even have offers such as booking a whole compartment for 1, 2 or 3 passengers at very attractive prices. In fact, that's one thing I learnt in the context of this thread and having a look at actual night train sites: how attractive these prices are. I see that there are even concepts in modern night trains like single cabins of the "pod" type.
These are all facts. Or let me be more precise: I am pretty certain that they are facts.
Now what's the point of our threads here, other than to while away the hours? To advocate at every possible occasion that everyone must watch and wait or to explore what possibilities there are to live life under the circumstances? To simply hear what others do and draw one's own conclusions for oneself like ok, that's something I might try, either now or in the near future, or no that's not for me, under no circumstances?
As to air in a train, airflows, mixing of internal and external air in systems on trains night or day, air flowing up and air flowing down, aerosols, virus loads, life expectancy of a coronavirus travelling on an aerosol and life expectancy of a coronavirus travelling in a droplet of watery substance, survival chances in dry air, ability to float in air for an extended period of time, trajectory, speed with which it is expelled and how far it will travel and its chances to hit you right in the eye, mouth or nose etc etc ... I recommend googling! Especially when one has the time.