The case is similar in England, Wales and Scotland [click here ;
In England and Wales, the Environmental Permitting (England and Wales) Regulations 2010 (Schedule 3, D2) provide a special exemption to rail companies from the need to obtain a permit to dispose of this waste onto tracks. Similar exemptions are available in Scotland and Northern Ireland under their Waste Management Licensing regimes. This waste is deemed to be
low-risk waste that the Government has determined requires only light touch regulation.
The...
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Rail companies must register the exercise of the exemption with the Environment Agency in England and Wales and in the case of Scotland and Northern Ireland registration is with the Scottish Environment Protection Agency (SEPA) and the Northern Ireland Environment Agency (NIEA) respectively. Rail companies must also ensure that the quantity of waste deposited per discharge must not exceed 25 litres, however there is no cumulative limit and therefore tens of thousands of litres of sink and toilet waste are discharged onto Britain's railway tracks every year.
"Most newer train carriages contain effluent retention tanks that are emptied regularly at maintenance depots, however older carriages including those of the InterCity model 125 still discharge waste onto tracks. Rail operators have argued that a comprehensive retrofitting of the older stock would be disproportionate to any health risks and prohibitively expensive. Prompted by health and safety concerns, research carried out by the Rail Safety and Standards Board in 2004 concluded that the health risks to rail workers were considered low and normal hygienic practices could mitigate risks of any exposure to microbiological contamination.This affects a minority of trains in service today as toilets that flush onto the track are being phased out as new rolling stock is introduced.” However, it is possible that a significant percentage of the carriages that still discharge waste onto tracks may be in service for up to a further ten years. This would continue to impact routes across the country, a consideration the Commission is likely to take into account in any investigation.

For this query, here is an article of development of new alloy for preventing degradation of rails due to toilet discharge.[click here
Nearly Rs.4.4 billion ($89 million) is spent annually on replacement of rails withdrawn prematurely due to corrosion.
A new alloy steel developed at the Indian Institute of Technology in Kanpur (IIT-K) has provided the answer to one of the major problems faced by the Indian railway system - corrosion of rails by toilet discharge. Field trials are in progress on use of NCC rails (Nickel, chromium and Copper - alloy...
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rails) in Vijayawada -Chennai route of Vijayawada divn of SCR. So far performace is satisfactory.
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