Hi,
Triggers can be used for a heap of different things, like other trains will wait for a trigger to be 'touched' or 'activated' by another train, to display html browser windows in game with say 'instructions' on what to do in the session. To check to make sure that 'something' has been done in the session, there are a lot of things that triggers can be used for.
(Making this up as I go...

)
Very quickly though, lets use the example, a train (train 1) will wait for a trigger to be 'touched' or 'activated' by another train (train 2) upon exiting an industry. Lets say that train 1 is waiting in a siding to be loaded with lumber at a sawmill 'after' a delivery of logs done by train 2.
First thing, you need the Driver Command 'Wait For Trigger' do a search on the DLS for
WaitForTrigger or kuid
57344:80001:2 and you should find it. Download and install.
Once done you have to enable the new command, load you layout/session that you want to use the command in, click on
Edit Session Rules button (move your mouse cursor to the top of the screen, it's the middle button), now select Driver Command and click the Edit button, scroll the list of commands to find Wait For Trigger. Place a green tick beside it and click on the 'tick' button to confirm, click on the 'tick' button to confirm and close the Edit Session Rules window, now its enabled and you can use it with your drivers.
Place the trigger on the track where the train will 'touch' or 'activate' it, in my example I would place the trigger on the track that exits the sawmill where logs are dropped off, name the trigger so you know what it is, in my example it would be something like
T-SMExit-01 where T = Trigger, SMExit = SawMill Exit & 01 = if there will be more then 1 trigger, then more numbers used.
Now in my example, I would place a train in the siding and call it train 1, place another train approaching the sawmill already loaded with logs and call it train 2.
Once again, click on
Edit Session Rules button this time select the Driver Setup and click Edit button, a list of drivers will be displayed with their trains, in the driver command bar for train 1 I click the small green arrow, highlight Wait for Trigger, highlight T-SMExit-01 now list of trains is displayed, I select train 2 from the list. From here the driver can be commanded to do whatever he has to do normally, as for train 2, he can be commanded as normal to do what he has to do, when finished, click on the 'tick' button to confirm and click on the 'tick' again to confirm and close the Edit Session Rules window.
What will happen in Driver? Well, the trigger will be invisible, train 1 will just sit and wait for the trigger to be 'touched' or 'activated' by train 2 before doing anything, train 2 on the other hand will work his way to the sawmill and unload the logs, when unloaded he will exit the sawmill (maybe to reload at a forestry) and 'touch' or 'activate' the trigger which will cause train 1 to begin his work to load lumber and do what he has to do.
Hope all that made some sense, done very quickly off the top of my head, but if you need more help just ask...
