When I re-wired my layout for DCC seven years ago, I went with 22AWG solid core feeders, six inches long on each and every piece of rail.
I don't solder to the sides of the rails (both C55 and C40), but to the bottom of the rail foot. I bend a 90deg. turn in the wire and squash it flat with my flat-nose pliers, then tin it, and solder it to the bottom of the rails both holding it and heating it with my resistance soldering tweezer, fluxing with Supersafe Gel Flux from H&N Electronics.
To keep them from being pulled off after the rails are re-ballasted, I CA the feeders into their respective holes drilled through my splined Masonite subroadbed using gap-filling CA and Accelerator, which because the layout is sectional and portable, is essential IMHO.
At shows, when buckled up to my friend and fellow crazy Gregg Cudworth's modules, which also have a feeder every piece of rail, we run up to a total of eight trains simultaneously on the mainlines, make up trains at Echo Yard and switch various industries on Gregg's sections, running two dozen or more engines at the same time, with no signal loss or any other electrical problems whatsoever.
So, in my 7 years of practical experience, 22AWG solid copper feeders (on every piece of rail) has been trouble-free.
Cheerio!
Bob Gilmore (finally recovered from 5 weeks of being ill!
)