The researchers found that air movement is slowest in the middle part of a train carriage. “If an infectious person is in the middle of the carriage, then they’re more likely to infect people than if they were standing at the end of the carriage,” said de Kreij. “However, in a real scenario, people don’t know where an infectious person is, so infection risk is constant no matter where you are in the carriage.”
Many commuter trains in the UK have been manufactured to be as cheap as possible when it comes to passenger comfort — getting the maximum number of seats per carriage. In addition, most commuter trains recirculate air instead of pulling fresh air in from outside, since fresh air has to be either heated or cooled, which is more expensive.
So, if it’s impossible for passengers to know whether they’re sharing a train carriage with an infectious person, what should they do to keep themselves safe? “Space out as much as you reasonably can — physical distancing isn’t the most effective method, but it does work when capacity levels are below 50 percent,” said de Kreij. “And wear a high-quality mask, which will not only protect you from COVID-19, but other common respiratory illnesses.”