Railway food is almost always "disappointing", since it can never measure up to the quality of home-cooked food. So there is nothing new here. Further elaboration (if possible) will make this tip more useful.
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1. How is it disappointing?
2. Are other Rajdhanis better? If so, which one?
3....
more... Has the food quality gone down recently?
4. What were you expecting, that led to your disappointment? Details needed. Was the food cold? Was it stale? Too much salt? Too watery? Generally not tasty? Roti was hard?
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Thanks.