The OP's HANDS were probably "dirtier" microorganism-wise and a lot more likely to spread any sort of pathogens than those off-color sheets.

When a night train arrives at it's destination, all sheets are taken away and then the wagons are cleaned>

I've been on night trains where the sheets are snatched off well before arrival but yes they always collect the sheets- never saw an unwashed sheet on night trains.

Although I do remember one train in Vietnam that ran from Hanoi to Saigon that I boarded at the half way point and the sheets had not been changed from the previous occupant. Of course, you were supposed to sleep in your clothes, one reaon I like the Dreamsack is that I can take most of mine off under the coves.