The term 'RAIL NAIL' is a combination of two words, 'RAIL' and 'NAIL'. RAIL refers to a metal bar or beam used in the construction of railway tracks, while NAIL is a thin metal SPIKE used to join two pieces of wood together. The crossword solution for this term is 'SPIKE', which is a metal fastener with a sharp point at one end and a flat head at the other. It is used to secure objects to walls or other surfaces, and is commonly used in the construction of railway tracks.