Complex sentence: Extensive narrow-gauge networks were constructed in many parts of the world for these purposes; for example, mountain logging operations in the 19th century often used narrow-gauge railways to transport logs from mill sites to market.
Split into simpler sentences: Extensive narrow-gauge networks were constructed in many parts of the world for these purposes. For example, mountain logging operations in the 19th century often used narrow-gauge railways to transport logs from mill sites to market.