Re: Solid Illustrations and models..
Func_illusionary are not-solid enities, usually they are used to make visible but passable trough things, like light beams....
I think, but i may wrong, that the wagon connector are made by models, using the item_generic entity. Since those models are not solid, you have to use the clip texure to nearly cover the shape of the model, so the player can walk over it.
Or, better, use a func_wall entity covered with a metal texture, with rendermode "texture" and Fx amount to 0 (zero).
P.s. yes, the item_generic model for the wagon connector is train_coupler.mdl