Transparent fences/bars?

Textures with transparent parts will only show up as transparent if you place them as a Mid-Tex on a 2-sided Lindef.

If they're showing as black, it's likely you placed them on a 1-sided Linedef (i.e. a line with no sector behind it). It's not showing as transparent because there is nothing behind it to show. 

In addition, if the sector is taller than the fence texture, let's say 144 tall, then flag the linedef as lower unpegged. Else the fence would render under the ceiling. Of course, another way to render the fence on the floor would be with texture offsetting.