One of the important moves while implementing SAFe is identifying Value Streams and the Agile Release Trains (ART), both of them carry a critical role and can be TERMED as the backbone of the scaled agile model. A value stream is a sequence of activities that an organization undertakes to deliver on a CUSTOMER request, and being more specific - A value stream incorporates all activities undertaken from START to end for a definite product or service in order to provide business value. Value streams are typically cross-functional, cross-organizational, and cross-geographical. When we develop systems to support value streams or deliver value directly, we do that with an Agile Release Train (ART). The ART contains all the right people, and all the skills needed, to deliver some or all of a value stream.

“ARTS are cross-functional and have all the capabilities—software, hardware, firmware, and other—needed to define, implement, test, and deploy new system functionality. An ART operates with a goal of achieving a continuous flow of value” – ScaledAgileFramework.com

An ART is a virtual organization that includes all of the people, with all of the requisite skills, to deliver value. After all, people do all the work, and we realize value with people in ARTs.

There are also ARTs that are physical organizations. ARTs are designed in such a way that each ART can deliver a set of capabilities, and in some cases, release independently of other ARTs.

A Value Stream can have many ARTs WITHIN it.

 Are there any reasons that Scrumban would not work with SAFe? 

SAFe actually is a Scrumban arrangement kind of. In the earlier version of SAFe, Scrum teams worked at the Team level, within the larger iterative Program