'rail-friendly' originates from Modern English as a compound of 'rail' + 'friendly'. 'Rail' developed in Middle English from forms like 'reile' or 'reile' derived from Old French (e.g. 'reille') meaning 'bar' or 'railing', and later came to be used for railway-related senses. 'Friendly' comes from Old English 'frēondlic' (from 'frēond' meaning 'friend').

'rail' changed from Middle English 'reile' (meaning a bar or railing) into the modern English 'rail' and acquired railway-related senses after the advent of railways in the 19th century; 'friendly' evolved from Old English 'frēondlic' through Middle English into modern 'friendly'. The compound 'rail-friendly' emerged in modern English usage to describe things favorable to rail transport.

Initially, 'rail' referred to a physical bar or railing; after the 19th century it acquired the sense related to railways. 'Rail-friendly' therefore originally combines those elements literally, and has evolved into a policy/planning and design term meaning 'supportive of rail transport'.