The calculated distance (air line) between Gonbad-e Kāvūs and Mahdishahr is approximately 146 miles respectively 236 kilometers.
By car or train, the actual journey to Mahdishahr is certainly longer, as only the direct route (as the crow flies) between Gonbad-e Kāvūs and Mahdishahr has been calculated here.
Air line (approximately)
236 kilometers
127 nautical miles