The calculated distance (air line) between Tuen Mun and Sau Mau Ping is approximately 18 miles respectively 28 kilometers.
By car or train, the actual journey to Sau Mau Ping is certainly longer, as only the direct route (as the crow flies) between Tuen Mun and Sau Mau Ping has been calculated here.
Air line (approximately)
28 kilometers
15 nautical miles