Can anyone explain how the different types of signals work? I haven't been able to figure it out yet. :[
You may take a look at sections 3.3, 3.4 and 4.1 in the wiki page: http://en.wiki.simutrans.com/index.php/Traffic_congestion_control (http://en.wiki.simutrans.com/index.php/Traffic_congestion_control)
The documentation has some pages about those: Starter Guide (http://docs.simutrans.com/#srm_link_sg).