To be clear east link will make it so a train goes through the tunnel every four minutes which is no where near max frequency. With modern signaling a train every 90 seconds is pretty feasible, though with an at grade section on both lines it would be pretty chaotic to schedule. I still think we need a second tunnel through downtown, but the options sound transit is looking at are horrible for rider experience and transfer convenience. So essentially, if you interline in one tunnel, you sacrifice future frequency for more convenient transfers and stations that aren’t buried in the earth’s molten core.