Double-track railway - Wikipedia

Quadruple-track railway - Wikipedia

File:Double-headed rail.jpg - Wikipedia

Railway Signals [Transport Fever 2 Wiki]

Trans-Siberian Railroad, History, Map, Geography, & Facts

Single-track railway - Wikipedia

Bilevel rail car - Wikipedia

So I decided to simplify Wikipedia's map(s) of double-track-railway-headedness per country (Source(s) in comments). Please tell me if I messed up, I'm partially colorblind :( : r/MapPorn

Main Line (Long Island Rail Road) - Wikipedia

New York City Subway - Wikipedia

Climate Change and Rail - Empire State Passenger Association

Double-track railway - Wikipedia

Quadruple-track railway - Wikipedia

Istanbul–Ankara railway - Wikipedia