Banjo spikes are little L-shaped pieces of wire that old-time banjoists, especially, drive into their banjo fretboards, underneath the fifth string (sometimes called the short, drone, or thumb string). They use these spikes like permanent capos for the 5th string — just tuck it under the spike to raise its pitch, usually in combination with a regular capo on the other four strings.
So, here’s the thing: They’re called “spikes” because they’re literally railroad spikes — used by model railroaders to hold down their HO-scale model train tracks. Banjoists have to buy them at hobby-train supply shops.
If you like your metaphors straight up, and no chaser, this is your poison: That banjo string is the lonesome old Long Steel Rail. Sometimes old-time banjoists die with a teeny-tiny little hammer in their hand, trying to beat that itsy-bitsy steam drill …
For vivid, multi-page instructions on installing railroad spikes in your banjo, see Richie Dotson’s BanjoResource.com.