Most of the guests will be forced to ride Mine Train via the Standby line because there aren't enough fast passes in a 12 hour park day to give one to every guest.
If you even conservatively assume there are 60,000 guests in MK on a day, and 40,000 of those want to ride Mine Train, and the park is open 9am to 9pm, that means Mine Train would need to have a fast pass guest throughput approaching 3,500 guests per hour, not counting any standby riders. (For comparison, BTMRR's total per-hour throughput is estimated at 2,400 riders).
So if they do not "tier" in MK, then all the Mine Train fast passes will be booked 60 days out by resort guests. Late booking resort guests and offsite guests will only get passes if 1) Disney holds passes and releases them in stages or 2) other guests cancel their rides. Otherwise, welcome to the Standby line.
This is why they put in tiering at Epcot - because given the choice, every single guest coming through the doors wanted to book Soarin and Test Track, and there simply is not sufficient capacity for this. So you can only pick one.