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+---
+type: reference
+last_update: 2019-07-03
+---
+
+# Merge Trains **[PREMIUM]**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/9186) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.0.
+> This feature is disabled by default, but [can be enabled manually](#enabling-merge-trains).
+
+[Pipelines for merged results](../index.md#pipelines-for-merged-results-premium) introduces
+running a build on the result of the merged code prior to merging, as a way to keep master green.
+
+There's a scenario, however, for teams with a high number of changes in the target branch (typically master) where in many or even all cases,
+by the time the merged code is validated another commit has made it to master, invalidating the merged result.
+You'd need some kind of queuing, cancellation or retry mechanism for these scenarios
+in order to ensure an orderly flow of changes into the target branch.
+
+Each MR that joins a merge train joins as the last item in the train,
+just as it works in the current state. However, instead of queuing and waiting,
+each item takes the completed state of the previous (pending) merge ref, adds its own changes,
+and starts the pipeline immediately in parallel under the assumption that everything is going to pass.
+
+In this way, if all the pipelines in the train merge successfully, no pipeline time is wasted either queuing or retrying.
+If the button is subsequently pressed in a different MR, instead of creating a new pipeline for the target branch,
+it creates a new pipeline targeting the merge result of the previous MR plus the target branch.
+Pipelines invalidated through failures are immediately canceled and requeued.
+
+## Requirements and limitations
+
+Merge trains have the following requirements and limitations:
+
+- This feature requires that
+  [pipelines for merged results](../index.md#pipelines-for-merged-results-premium) are
+  **configured properly**.
+- Each merge train can generate a merge ref and run a pipeline **one at a time**.
+  We plan to make the pipelines for merged results
+  [run in parallel](https://gitlab.com/gitlab-org/gitlab-ee/issues/11222) in a
+  future release.
+
+## Enabling Merge Trains
+
+To enable merge trains at the project level:
+
+1. Visit your project's **Settings > General** and expand **Merge requests**.
+1. Check **Allow merge trains**.
+1. Click **Save changes** button.
+
+![Merge request pipeline config](img/merge_train_config_v12_0.png)
+
+## How to add a merge request to a merge train
+
+To add a merge request to a merge train:
+
+1. Click "Start/Add merge train" button in a merge request
+
+![Start merge train](img/merge_train_start_v12_0.png)
+
+## How to remove a merge request from a merge train
+
+1. Click "Remove from merge train" button in the merge request widget.
+
+![Cancel merge train](img/merge_train_cancel_v12_0.png)
+
+## Start/Add to merge train when pipeline succeeds
+
+You can add a merge request to a merge train only when the latest pipeline in the
+merge request finished. While the pipeline is running or pending, you cannot add
+the merge request to a train because the current change of the merge request may
+be broken thus it could affect the following merge requests.
+
+In this case, you can schedule to add the merge request to a merge train **when the latest
+pipeline succeeds**. You can see the following button instead of the regular "Start/Add merge train"
+button while the latest pipeline is running.
+
+![Add to merge train when pipeline succeeds](img/merge_train_start_when_pipeline_succeeds_v12_0.png)
+
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->