48 springs, sized perfectly for zero friction addition when used to add power pickup on 1.5mm axles (N scale and P4 standard size).  They are a delight to use. – Simply remove one wheel, add the spring and replace the wheel-set, making sure you note which wheel is ‘live’ to the track. Drill a small hole in the floor of the coach/tender/caboose etc, and solder a bit of fine wire to the tag end on the spring. Connect the wire to your flickerfree or loco pickups etc. You will be delighted these friction-free pickups. They will add nothing to the drag at all and will always be able to give perfect pickup as long as you keep track and wheels reasonably clean.