The traditional method of disposing human waste from trains is to deposit the waste onto the tracks or, more often, onto nearby ground using what is known as a hopper toilet. This ranges from a hole in the floor to a full-flush system (possibly with sterilization). The hole in the floor (also known as a drop chute toilet or direct flush toilet) system is still in use in many parts of the world, particularly on older rolling stock. The principal disadvantage is that it can be considered crude or unhygienic and dangerous to health and the environment – it litters the railway lines and can convey serious health risks when the train passes over or under a navigable waterway. Passengers may be discouraged from flushing or using toilets while the train is at a station or standing at a red signal. To enforce this limitation, toilets may be automatically locked when the train pulls into a station or stops at a red signal. In the United States, railway employees were required to lock the toilets closed whenever a passenger train stood in a station or at any other location designated by instructions in the timetable. Toilets would promptly be unlocked upon departure.