They have to be cleaned at designated stations. I have seen in some trains the staff appointed, cleaning them. In CSMT - HWH kolkata mail, I saw it being cleaned at Etarsi, in TPJ - SGNR HS,it was cleaned at Pune, and many other stations. In both the trains I was in AC class, so the situation might be different in non ac class. The primary truth is that along with passengers the staff also needs to be kicked up to retain the cleanliness of trains.