Release trains are used to package multiple products with different versions to a larger package. This is called Release Train. The Spring Framework and other, larger projects are using this method to bundle smaller artifacts to a larger package. A release train has a release date, on which all releases will be bundled released together.
In ninkik you can assign each release to an optional release train.
In the following diagram you can find the release trains
Release Train 2020 has the releases 1.1.0-STABLE of the product My product #1 assigned and the release 2.0-STABLE of the product My product #2. The Release Train 2021 only contains a future release 3.0.