Historical clue formulations

• Some underground networks — 2025-08-28
• London and Budapest have had them since 1863 and 1896, respectively — 2025-02-15
• Urban areas, informally — 2024-05-24
• Underground networks — 2019-06-06
• Subways — 2018-07-04
• Subway systems — 2014-08-31
• Some urban transit systems — 2011-09-07
• Underground systems — 1998-02-01
• Subways — 1995-04-02