Semnan is straightforward to reach because it sits on the main Tehran-Mashhad road and rail line. Buses between Tehran, Damghan, Shahroud, and Mashhad stop here regularly; from Tehran the road trip is usually around 3 to 4 hours, traffic permitting.
The railway is useful if you are travelling the long east-west route across northern Iran. Semnan station is served by trains on the Tehran-Mashhad corridor, though schedules vary and should be checked locally. From Semnan, shared taxis and minibuses connect to Mahdishar and nearby towns.