What was needed to make railway sleepers?

Correct answer: Strong timber

The direct answer is D, strong timber. Railway sleepers are the supports placed beneath railway rails. They must bear the weight and repeated movement of trains, keep the track aligned and remain durable under weather and pressure. Step by step: rails need a stable base; sleepers hold the rails at the correct distance; strong wood was commonly used historically; railway expansion therefore created heavy demand for suitable timber. Option A, glass, is wrong because glass is fragile and cannot serve as the normal structural sleeper material. Option B, cotton, is a soft fibre used for textiles, not for supporting railway rails. Option C, salt, is a mineral and cannot perform the required supporting function. Option D is correct because durable timber was needed for sleepers. Modern railways may use concrete or steel sleepers, but this historical question concerns the large colonial demand for strong wood. Memory cue: sleeper means a strong support under rails, so think timber.