If there are any trains on a layout, then one of them is always designated the "selected train." This is the one
To select a train: (a) click any car in the train, or (b) choose it from the Train menu.
At any given time there may be at most one "selected car" on the layout. If a car is selected:
a
green X is shown on the car topTo select a car: (a) click its image on the layout, or (b) click its icon in a train window. When you select a car, you also select its train.
To unselect the car: click a blank space on the layout, or the blank space behind the train in the train window.