2×2 multi-floor elevator design

Pros of this design:
+ Any number of floors, with any distances between individual floors.

+ Being able to summon elevator to any floor.

+ Being able to go straight to any arbitrary chosen floor.

+ Being able to detect if the carriage is present on the current floor (useful for safety doors, bells and such).

+ Fairly small footprint.

+ Can be extended up or down at any time.

Cons:
– This design can't have a second "ceiling" carriage, it's incompatible with the work logic.

– Current implementation is a bit slow – it needs to send a redstone signal to the target floor and back before the carriage starts moving.

– Shaft walls aren't 100% solid, some redstone will be visible.