To split a train into two separate ones, you first have to select the train. Then you navigate to the node where you like to split the trainrun. Inside the node the trainrun has to have a transition. Press CTRL key and click with the mouse the "stop / non-stop toggle button". The trainrun gets split into two trains.
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To combine two trainruns, you have to select one of the two trains. Then you have to navigate to the node where the trainrun ends (or starts). Now you can draw the new transition similar to creating a connection - but you have to press CTRL key and it must be held pressed as long as you are drawing a new transition. Once you finish drawing the new transition, both trains will be combined to one single trainrun. Please have as well a look into Create Connections.