Some utter make believe stuff on here.
Yes air is recirculated
Yes some fresh air is brought in from outside (but usually a smaller % as it’s less efficient to have to keep cooling new air). There are regulations on this, usually air flow per person or square ft and it’s brought in by a dedicated intake with its own filters.
Yes aircraft bring in fresh air, some newer ones even bring in quite a bit, but there’s always recirculating happening too.
Trains and building systems will have multiple “cassettes” for each space, these are essentially “cold radiators” that get a flow of coolant (water or a chemical) from another radiator outside - they don’t actually have a big air pipe that goes outside! So the big vent above your head blasting out cold air will only be taking in air from your immediate surroundings anyway: well within the sneeze range of an infected person.
Trains use simple ducting to push this cold air out to various bits of the carriage, but again only so far from their zone. The intake for a cassette will usually just be one vent at the highest point and easy to spot.
They do have filters, and they filter out most stuff. Virus tend to be smaller than filters can handle, but viruses do tend to clump together so a decent HEPA filter will likely catch them.
The most likely way you’ll catch something is contact or someone coughing on/near you. People who travel whilst ill and who don’t practice basic hygiene like coughing/sneezing into a tissue should be shamed by everyone around them- ignoring it just lets them keep spreading.
