Trains run twice a month from Quetta in Pakistan, leaving Quetta on the 1st and 15th every month. The journey takes around 11 hours and costs around 1000 Pakistani rupees.
There is also a flight operated by PIA from Zahedan to Quetta. Check www.piac.com.pk for more information.
The railroad from Bam/Kerman to Zahedan was finally inaugurated in June 2009, with traffic started in January 2012. There is a direct Tehran-Zahedan train, which takes about 24 hours and is formed of couchette sleepers. More information can be found on the Iranian railways website.
Pakistan - this is a likely candidate for why you're here in the first place, but make sure you've got a visa before coming, as the consulate here no longer issues them. Mashad also has a Pakistani Consulate where you can get visas there, which is the nearest place to pick one up. Also Tehran has a Pakistani Embassy.