An 18 gauge, ½" penny nail can be utilized. Nails can be bought at any good hobby shop. Just ask for rail road track nails. We do not support driving nails into the track as the track could break or the nails could cause short circuits. There is no provision in the track to insert nails.
HO refers to the scale of most ER cars and the original root of the reference is in Lionel model train sets. In these sets, there existed "O" gauge or scale and "Half-of O" gauge. "Half-of O" was shortened to HO and that's where the name came from.
In general, we do not recommend your children play with our "vintage" toys as toy standard and regulations which we live today were non-existent years ago. For example, lead in paint regulations and rattle gauges. Also, the CPSC did not exist until the 1970's.