Question

why have the subjects like Railway defence and Foreign Affairs been including in the union list and not in the state list give reason

Answer 1

it is because these subjects are of the whole country not in a particular state.
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Answer 2

The subjects like railway defence, foreign affairs, banking, etc. have been included in the Union List because these subjects are of national importance and require a uniform policy for execution.

In the State list, the subjects are of state and local importance, such a police, trade, commerce, etc.

In short;

Railway defence and Foreign affairs are of national importance, so they are in the Union list and not in the State list which are of local importance.