The HO gauge railway spikes work fine but they are now hard to find. You do not want the ones with round heads. You want the old style with the off-set head so you can tuck the 5th string under it. If you'd like a dozen or so, I still have several hundred of the little suckers. PM me and I'll mail them out to you (US only) for a dollar. Off course, the postal department may not like sharp metal objects being sent through the mail but I generally surround them with masking tape.

Some further words of caution, pre-drill before hammering the spikes in, and don't hammer them in with a sledgehammer.

Cheerily,
Charley Noble