For me, UTC always worked better in OpenGL mode. On the old desktop, DirectX mode would not even work at all. On my normal laptop that I have Trainz on these days, DirectX mode would work with UTC, but it had the behavior of OpenGL on the desktop, e.g. when you would zoom out, the water under the ground and other similar stuff, such as the bases of objects, would flicker with the ground. With TRS2004, DirectX works best. One bad thing about OpenGL is that it does not support reflection effects, in my experience. This means that translucent loco and rolling stock windows that have a reflection effect will be displayed with no reflection and no alpha channel. DirectX supports reflection.
Which graphics mode you use probably depends both on your computer and on which Trainz version you use. On a side note, of course, the stuff that I just mentioned above probably applies to any 3D program that uses Auran JET as its graphics engine, as Trainz uses Auran JET.
Regards.