The term 'Plane or Rail Variety' can be summarized as a type of transportation that can be either by plane or by rail. The crossword solution for this term is 'MONO', which is a four letter word. Mono is a prefix meaning 'ONE' or 'SINGLE', and in this context it refers to the fact that there is only ONE type of transportation being used. This could refer to a SINGLE mode of transportation, such as a train or an airplane, or it could refer to a combination of both, such as a train and a plane.