Observe almost any railway track in Europe and you will likely notice a line of cement tiles running all along the track-bed. These are the covers of ducts that carry electrical cables used for running the railway.
I decided I would add some to my layout and set about creating some designs which could be printed on my 3D printer. The first question was, "How big are they?" I spent some hours browsing through the online product catalogs of European manufacturers who specialize in making the ducts and various trackside accessories for the rail industry. They come in almost every size imaginable!
I settled on making ducts 40cm wide, and designed them with gaps between the edges and covers.