The Wikipedia article for the Shuttle Carrier Aircraft mentions an escape tunnel:
The avionics and engines were also upgraded, and an escape tunnel system similar to that used on Boeing's first 747 test flights was added. The flight crew escape tunnel system was later removed following the completion of the Approach and Landing Tests (ALT) due to concerns over possible engine ingestion of an escaping crew member.
What is this escape tunnel? Who was intended to escape: someone in the orbiter, or someone in the SCA? Are there pictures?