How can we reserve 2 taxis or a mini bus for 8 senior persons with luggage from Mashhad train station to Bajgiran.?
I doubt you can book a taxi in advance (though my experience of travelling in Iran is 14 years old...).
I would suggest contacting an Iranian travel agency to organise a minibus.
This agency seems to be recommended on other travel forums:
This one states it provides transfer services:
A few others to try:
If they can't help you directly, i'm sure they'll put you in contact with the right agency. There are more out there!
Iran is amazing, i yearn to go back. Enjoy!