The calculated distance (air line) between Ḩalfāyā and As Salamīyah is approximately 31 miles respectively 49 kilometers.
By car or train, the actual journey to As Salamīyah is certainly longer, as only the direct route (as the crow flies) between Ḩalfāyā and As Salamīyah has been calculated here.
Air line (approximately)
49 kilometers
27 nautical miles