When would a railway be considered a heritage railway?

Context: A heritage railway or heritage railroad is a railway operated as living history to re-create or preserve railway scenes of the past. Heritage railways are often old railway lines preserved in a state depicting a period in the history of rail transport. The British Office of Rail and Road defines heritage railways as follows:...'lines of local interest', museum railways or tourist railways that have retained or assumed the character and appearance and operating practices of railways of former times. Several lines that operate in isolation provide genuine transport facilities, providing community links. Most lines constitute tourist or educational attractions in their own right. Much of the rolling stock and other equipment used on these systems is original and is of historic value in its own right. Many systems aim to replicate both the look and operating practices of historic former railways companies.

Answer: Generally speaking, a railway can be considered a heritage railway when it assumes the appearance, characteristics, and operations of that railway from a period(s) in its past. These railways are often old and have been intentionally preserved to maintain its operating practices from prior times, purposely keeping equipment and processes original to a period in the past. Many of these heritage railways are preserved for educational purposes or as tourist attractions.