The correct answer is A. 30 kmph.
The headlight of a train is an important safety feature that allows the train to be seen by other vehicles and pedestrians. In the event of a headlight failure, the train must reduce its speed to 30 kmph to ensure the safety of all road users.
Option B is incorrect because 40 kmph is still too fast for a train with a failed headlight. Option C is incorrect because 60 kmph is even faster than 40 kmph and would be very dangerous for a train with a failed headlight. Option D is incorrect because the maximum speed of a train in the event of a headlight failure is 30 kmph, not its normal speed.