Shake Shack has TWO lines, not one.
The register on the left is for ordering just custard and drinks and the one on the right is for ordering everything (including custard and drinks). There are signs above the registers that indicate this, but very few people seem to know about this so there is always just one big line that everyone waits on.
I feel like a jerk when I bypass the long line and just walk right up to the register on the left and order my custard, because most people probably think I'm cutting the line, but that is how the Shack is set up. So please people, if you are there to just order custard or drinks, don't stand on the long friggin line, just walk right up and order from the left register.