On Thursday, October 25, 2018 at 1:16:32 AM UTC-4, Tosspot wrote:
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There is no real reason 'track' fork ends can't have a derailleur, they
just don't. The advantages of each style are well known, but basically
single/fixes/hubs like a track end for 'easy' chain adjustment, every
body else would use forward facing or vertical for ease of getting the
rear wheel off/on. In my case, I use a vertical, with one of these,
albeit generic,

https://surlybikes.com/parts/drivetrain/singleator

Which works very well imho, giving me the perceived advantages of a hub
drive, without the hassle of chain tensioning or the fudge or a short
reach derailleur. Yes Andre, I know, I can live with it.

See also

https://www.slowtwitch.com/Tech/Drop..._101_3354.html

Looks a lot as though that Sinleator was developed from either a V-brake caliper or an old school straight arm cantilever brake arm.

Cheers