If I were the conductor, I would simply say that one adult has to stay with kid(s). Remember, conductors are the bosses of the train, and have the power to kick off passengers who challenge their authority, which may occur in the middle of the night in some small hick town with no taxi or Uber service.I don't have experience with the 4 in one room policies, but my idea would be to claim your children are ill behaved, prone to motion sickness with projectile vomiting, won't sit still even at night, loud screamers, list out other negative behaviors, etc. and if they are required to ride in coach with no supervision they will cause mayhem, noise, and complaints from all the other coach passengers.