The answer should be 'This is the 7th train to Delhi'. How can I properly ask this question ? The keyword in the answer is '7th'.
2 Answers
You can ask the questions in various forms such as-
Is this the first train to Delhi? Was this the last train to Delhi? Is it the only train to Delhi.
You can provide some context before or after "This is the 7th train to Delhi", to give some context to your answer.
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1For each of your sample questions the answer is more likely to be a simple "yes" or "no" than "This is the 7th train to Delhi". Feb 20, 2019 at 12:43
A roundabout way of asking this would be: "How many trains have already departed for Delhi?", and then the answer would be "Six", and you'd have your answer implied.
The grammatically correct way to ask this would be: "In the order of trains to Delhi, what is its position?"