The term 'HALF A TRAIN TRACK' can be solved by the word 'RAIL', which is a four-letter word. A RAIL is a horizontal bar of metal, typically supported by posts or attached to a wall, used for various purposes such as forming part of a fence or barrier, or as a structural element in the construction of a building or bridge. In the context of a train track, a RAIL is a metal bar that forms part of the track on which trains run. Therefore, the term 'HALF A TRAIN TRACK' can be interpreted as referring to one of the two metal bars that make up a train track.