Late 60's artists like The Band, CCR, Burrito Brothers, CSNY and later acts like Steve Earle, Counting Crows and Ryan Adams can be grouped in this category. Some overlap here with 'Outlaw Country' modelled on the music of Johnny Cash, Willie Nelson and Merle Haggard. The Turnpike Troubadours and Chris Stapleton are flying the flag these days. The UK has a wealth of talent with Case Hardin, Dreaming Spires, Danny & The Champions of The World riding high.
Bluegrass stands apart but is frequently found within Americana as the root instruments like Double Bass, Fiddle, Dobro, Acoustic Guitar and Mandoline feature heavily in songs. Alison Krauss and Union Station great exponent. Many great UK Bluegrass bands Coal Porters and Southern Tennant Folk Union to name but two..