Relations Between Union and States
(1) A State's executive power must not impede the Union's executive power, and the Union can direct a State as necessary to prevent that. (2) The Union can direct a State on constructing/maintaining communications declared nationally/militarily important. (3) The Union can direct a State on protecting railways within it. (4) If following such a direction costs a State more than its normal duties would have, the Union must pay the State the agreed (or arbitrator-determined) extra cost.
Beyond just ensuring compliance with laws (Article 256), this lets the Union actively direct states on specific practical matters of national importance - protecting key infrastructure and railways - while also guaranteeing the state gets compensated for any genuine extra cost of complying with such a direction.
This is why the Union government can direct a state on maintaining a highway or railway line designated as nationally important, while also being obligated to reimburse the state for any additional expense that direction causes beyond the state's normal costs.