The release train branching strategy is similar to what is sometimes referred to as late branching or trunk based development. Essentially it implies that in your entire branch tree there is only one branch that is meant to be long-lived.

Consequently, there is only one way you can contribute to a product and have your code released, and that is to deliver your code to the mainline.

In a release train strategy, the mainline is on it’s way to production all the time, and therefore anything that isn’t meant for production as soon as possible, shouldn’t be delivered to the mainline.