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Let the distance between A and B be \(d\) km.
Time taken to go from A to B at 40 km/hr is \(t_1 = \frac{d}{40}\) hours.
Time taken to return from B to A at 10 km/hr is \(t_2 = \frac{d}{10}\) hours.
Total distance covered = \(d + d = 2d\) km.
Total time taken = \(t_1 + t_2 = \frac{d}{40} + \frac{d}{10} = \frac{d + 4d}{40} = \frac{5d}{40} = \frac{d}{8}\) hours.
Average speed = \(\frac{\text{Total distance}}{\text{Total time}} = \frac{2d}{\frac{d}{8}} = \frac{2d \times 8}{d} = 16\) km/hr.
Therefore, the average speed of the train during the two-way journey is 16 km/hr.
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