I mostly used 3½″ floppies back in the day but I also used 5¼″ floppies to some extent on Apple II's and TRS-80's. I was never exposed to 3″ floppy disks as used mainly on Amstrad microcomputers and their Spectrum +3.
So I'm familiar with the concepts of single-sided vs double-sided floppies, and single-density vs double-density floppies.
But in my recent explorations of the Spectrum +3 and its floppy disk formats and their image file formats, I've come across various mentions of "single-track" vs "double-track" and their abbreviations "ST" and "DT". But googling for more information on these terms, I haven't been able to find anything.
Is this double-track thing something specific to the Amstrad 3″ floppy drives? And what is it and how does it work? Was it a dead-end technology? Did it require different disks? Did it double the disk capacity?