Is it possible to add the glow you get from the intelligence, or a payload cart to a normal brush or anything else?
If you mean the team-coloured outline, that's already on Payload karts. I'm pretty sure that you can't put it on other entities though.
I believe there's a tutorial on how to do this on the valve developer wiki... http://developer.valvesoftware.com/wiki/L4D_Glow_Effect I think this it it.
Yeah, you can. Make a tracktrain, and parent something to that. Don't forget payload logic. And of course, the tracktrain can be parented to any other thing.
you take the base payload entity (func_tracktrain) and parent whatever model you want to have the glow, to that tracktrain. Then, add the payload logic. Remember, you do nto need to have path_tracks for the tracktrain entity at all. Also, this does not mean that the game mode is payload inherently. Lastly, parent the tracktrain to whatever you like, if you want it to be dynamic.