Click on an individual train to select it for management.
Click on a section of track and a list of all trains using that section will appear on the left of the screen. Clicking on a train in that list selects that train.
As new models of engine become available, or trains get worn out and slow down, you may want to upgrade your trains.
From the Engine Shed (4th icon at the top left) you can scan through the various engine models available. At the top right of that screen you can buy a new engine with the $ button, or upgrade a bunch at once with the =$= button. With the latter, you can either upgrade *all* your engines to a different model, or filter by Age or Suitability (Freight, Express, Mixed – determined by the model) to upgrade all engines in that class. You are refunded the remaining value of the old engine.
You can also select an existing single train and click on the Manage Train button (train with a pencil) to open a detailed individual manager. At the top right is a $ button to “Buy locomotive.” This new engine will replace the old one, again refunding remaining engine value.
Sometimes you want to manage a train’s route. Select a train and click on the little map button above the train information to make changes to the route. Each stop has a box on the left of the screen. Hover over one and click on the pad of paper icon to open the manager for that stop. You can click on individual types of good to cycle through preferred (green) banned (red) and normal (no added icon) pickup of that good at this stop. You can set min and max carloads to be picked up (total.)
While in route management, the right hand box can set the train route to be Automatic, Passengers and Mail Only, Freight Only, or Manual. Automatic is generally pretty good, but experiment! Restricted modes will pick up only those goods. Manual allows the player to set EXACTLY what to pick up and drop off at a Station. Click around, you’ll get the hang of it.
Trains will ONLY pick up goods that are in demand at one of their other Stations, and only as many goods as that/those stations have room for. If a train isn’t loading the goods you have specified or want it to, this is probably the reason. Cities have limited space for each type of good, and if another train is already on the way, the goods it carries count to that limit.