This is a stage you may not need. If it is important to you to have trains accelerate slowly when loaded by a heavy coal train but move faster when not loaded, or if you want double heading, then Trains are the mechanism to do this.
In Edit mode, click "create train" from the "trains" tab. It will need to be given a name, length & weight. You can then assign the engines used to haul it. If you are "double heading" then you will assign two or more engines.