This may seem at first to be an odd question but there is a convention that has been used for more than sixty years concerning this very subject. Model railway manufacturers the world over have all adhered to this convention resulting in when you put your train on the track and turn on the controller, the train will proceed in one consistent direction.
The convention states that when the furthest away rail is more positive than the nearest rail, the train moves to your left. Reversing the controller makes the furthest away rail more negative than the nearer rail, so the train moves to the right.
If your train does not adhere to this convention, you have a faulty train. This error could be due to a number of reasons. If the loco is brand new then there is a manufacturing fault and it should be returned to the retailer as being not fit for purpose. However, on older pre owned models, common reasons are that the magnet has become reversed or that the wiring to the motor has been reconnected incorrectly. If directionally controlled headlights are installed, it is important to ascertain whether the headlights and travel direction are in synchronisation. Reversing the wiring to the motor or lighting will remedy the problem.
Why is direction important? You really want all your trains to run the correct way for ease of operation in running, shunting and is of course it is absolutely essential for double heading or banking purposes. Also, as we shall see later in the Short Circuits series, control circuits utilising diodes will not work as intended on reverse wired locos. Don't end up going backwards. It's easy and it's worthwhile to get your direction correct.