Dust dont imply idle stay but unused berth while berth runs in dust driven terrain...Lichchavi with lil idle TAT @ Sitamarhi will show dust driven berth, which simply defines dust getting settled during run!!!
Further, u defined correctly the reason behind issue..bio-toilet..they been implemented without any sanitation facility @ pit-lines nor do anyone cares the same & remain choked...also traveler plays vital role in washroom sanitation..
Once SBC Raj washroom got choked & hence unfit, but was addressed by running maintenance staff & same run in approximately all M/E for SL coach even, which need to be informed about any possible choke!!!