I see 4 types in TRS2004. Let me see if I can tell what they do...

:hehe:
The green T-shaped trackmark is a trigger. Triggers are used to, well, trigger events in a script whenever a train passes over them or is within the trigger's set radius (set in the "Advanced" sub-menu). You can use them in rules in a session, I think. Their original purpose was in "Scenarios"--scripted Driver sessions designed to mimic real railway operations--that were introduced in Trainz version 1.3 (i.e. Trainz SP3). They were used to tell the scenario's script that a train had passed over them or was within the trigger's radius (as I just explained).
Other users are free to trout-whack me here if I explained something wrong, as, while I know what the regular (red) trackmark and the trigger do, I was just speculating on what the others do.

:hehe:
Regards.
EDIT (conflict): JCitron posted while I was still writing mine.

:hehe: I think he explains it pretty well.

Regards.