TranslucentLine

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208:TranslucentLine (lineid, amount, additive?, moreflags)

lineid: Line ID of lines to make translucent (0 for this line) 
amount: How translucent the line should be [0..255]. 
additive?: Whether this translucent line should use additive translucency or not (0 = normal, 1 = additive)
moreflags: Sets extended lineflags. See Line_SetIdentification for details.
This parameter only has a function when used directly in a map.

Sets the amount of translucency for all matching lines (including itself). If lineid is 0, it only sets the translucency of the line it is on. Like Line_SetIdentification, this special sets the linedef's id. Amount controls how opaque the line is. 0 is nearly invisible. 255 is opaque. Intermediate values are somewhere in between.

You can also use this special in an ACS script to change the translucency for lines whose ids match lineid.