The basic difference between Railway Mutton Curry and Bengali Kosha Mangsho is that the latter is a lot spicy than the former one, which has a mild flavor. Railway mutton curry is prepared with boiled mutton pieces, whereas mutton in kosha mangsho is nicely marinated with some curd and aromatic spices for about 8 long hours so that the spices are properly absorbed in the mutton. The mutton pieces were boiled before adding them in the curry just for the sake of convenience. When you are making a curry for thousands of people and that too in a train's kitchen, you just improvise on cooing methods.