Incidentally there is nothing stopping a non mobility challenged person from booking the lower level on long distance trains. You buy the lower level ticket and are entitled to occupy the seat. On a lightly traveled day, it can be extremely quiet as there is no through traffic. On western trains there is generally a door between the vestibule and the actual lower level compartment. So the noise of passengers hitting the restroom should be minimal. There is less sway, but more track noise, on the lower level. Also, since the lower level is a relatively small chamber, that means that if someone is snoring or talking loudly, that noise will not dissipate as quickly as on the upper level.