There are two kinds of these. If they are from Boat, they are crossing triggers and are used to activate the level crossings. The other ones, are just to "trigger" events. Here's an example:
The other kind of trigger is like a relay switch on a model railroad that will start other events once the trigger is crossed over.
You have a stub-ended terminal that you've pulled your train into locomotive first.
As you are pulling your train in, you cross over a trigger and pass it. This "tells" an other configured AI-driven shunting locomotive to couple on to your consist, and uncouple your locomotive from the wagons. Your consist, now empty of passengers, is hauled off to the coaching yards and awaits another train later on.
In the meantime, your locomotive, now driven by another AI driver, is sent off to the engine house for maintenance and when completed awaits a new run.
How to set this up? Beats me because I like to keep things simple, but this is basically what a trigger does.
To see the innards, edit the session rules and you'll see the triggers with sub-routines all lined up underneath them. They are indented to show they are "child" routines off of the triggered event.
I hope I 'splained this okay.