The calculated distance (air line) between Sint-Pieters-Leeuw and Mouscron is approximately 45 miles respectively 73 kilometers.
By car or train, the actual journey to Mouscron is certainly longer, as only the direct route (as the crow flies) between Sint-Pieters-Leeuw and Mouscron has been calculated here.
Air line (approximately)
73 kilometers
39 nautical miles