Inching towards fulfilling its announcement made in this year's railway budget, the Indian Railways on Friday said it has developed a prototype of hybrid vacuum toilet which uses only 500 ml of water for flushing.The early sample, a mix of vacuum toilet generally used in aircraft and bio-toilet or conventional one, had been fitted in one of the coaches in New Delhi-Dibrugarh Rajdhani for trial, the ministry said in a statement.Typically, a conventional toilet uses 10-15 liters of water per flush, while the vacuum toilet consumes only 500 ml of water per flushing."Water is a very precious natural resource. Therefore, this innovation shall save water to the tune of at least 1/20th of the quantity that is used in the current design of bio-toilets or conventional toilets," the statement said.The design was developed by the development cell of the Railway Board that combines the advantages of vacuum toilets and those of bio-toilets to create a new design of hybrid vacuum toilet.