The Middle Straight Track, or more commonly known as "A3" appeared in sets from 1999 to somewhere late in the noughties. The track is 7.2cm long, the length that complements a Medium Straight Track of 14.4cm to get to the length of a Long Straight Track of 21.6cm.
It appeared as a standalone 4 track pack in the 1998 foldout that came with BRIO products but I believe that hardly got any sales. Maybe people would not understand the difference between the Mini Straight of 5.4cm and this only slightly longer Middle Straight.
It might be that therefor it only shipped in sets when BRIO started putting out asymmetrical sets like 33105 The Cargo Express Set