Remember that the iron circuit also has to be nearly complete-- the gap between the pole piece that carrier the winding, and the coupler armature should be the only noticeble gap. Also, the magnet will not accuate wound coil couplers. Recall the 6o17 uncoupling track (0-27) is a sheet of steel with a dimple to hold the coupler coil, while the long edges are folded to form the two outside rails. Non-magnetic materials cannot be in this mag circuit, which must form a nearly closed loop.
Common non-magnetic metals include soft, bendable stainless steel (austenitic, iirc} of Gargraves stainless (the tinplate is okay), Atlas O rails (aluminum), brass rail, zinc or zamak castings, Common magnetic materials include tinplated iron or steel, soft iron, silicon-bearing iron, galvanized iron or steel, also chromed steel. Cobalt is magnetic, often found in permanent magnets, as is the alloy Alnico. German silver (cupronickel) rails are not magnetic.
Air is magnetic, however it is about a thousand times less so than the common materials used in making motors: soft iron (DC); silicon-bearing iron (laminated sheets for AC), In general air gaps must be kept as small as practical. Copper windings are not magnetic, however when carrying a current the magnetic fields in air gaps exert a force on the moving electrons, which they in turn exert a force on the copper wire carrying the current. Parallel wires carrying current in the same direction experience forces that repel the wires, hence coils are tightly wound and anchored.
It is fairly certain that with a plastic (non-magnetic) roadbed you are lacking a complete magnetic circuit, This must be supplied. A look at a factory-made uncoupling section may show how this can be done. Overheating should be prevented by using a push-button switch and a brief contact only.
--Frank
