Agile release trains are the most important things for scaled agile framework. You can think of Agile Release Trains as "Team of Teams". Multiple scrum teams working on a single objective is, collectively is called an Agile Release Train. Another very important aspect of Agile Release trains are incrementally develop, continuously integrating, continuously delivering, in at least one value stream.
Value streams actually represent a series of steps which an enterprise takes to implement and deliver a solution. A value stream contains people who do the work, systems they develop or operate upon, and the flow of information and materials. In a broad sense, these all things can be contained inside a value stream. I have covered value streams in a separate piece here.
Agile Release Train - Promotes Autonomy and Alignment
We are talking around 50 to 125 people, who have to work together for creating a solution or a service or a product. When so many people are working together even communication becomes challenging. Therefore, Agile Release Trains are cross functional and have all the capabilities. Members from all departments, needed to define, implement, test, deploy, release and operate solutions (if needed) join in one single team.
ARTs a.k.a agile release trains operate on some common principles. They are:
The schedule is fixed: The train starts will all teams, together on a specific day, as determined by a chosen PI (Program Increment) cadence.
A new system increment every two weeks: Each ART delivers a new system increment every two weeks. Post which the ART plans a system demo which provides a mechanism for evaluating working software. This has to be an integrated increment, combined from all the teams in an ART.
A fixed PI time box: All teams are synchronised on a similar PI (Program Increment) time box (8 to 12 weeks). Teams have a common iteration start / end dates and durations.
ART has a known and predictable velocity: Each ART is able to estimate how much new features can be delivered in a single PI.
ARTs are made up of multiple Agile Teams: Agile teams embrace the Agile Manifesto and SAFe values and principles. They apply Scrum, Extreme Programming, Kanban and other "built in quality" practices.
Dedicated People: All team members are dedicated to the cross functional agile team. They can have separate reporting hierarchies, it does not matter. But once they are part of the ART, they remain dedicated to this team.
Face to Face PI Planning: The ARTs plan their work in a large face to face PI planning session. Colocated team members have to come together to attend this session. For team members sitting across locations would need to attend over video conferencing modes.
Inspect and Adapt: Inspect and adapt sessions are held at the end of the PI and before the start of a new one. The current state of solutions are demonstrated and and improvement ideas are identified.
Agile Release trains thus are the most important aspect of SAFe. The entire value stream is alive because of them.