Short answer: Pure copper conducts best but lacks the strength and heat resistance for high-speed catenaries that combine high tension with anti-softening requirements. Copper-silver (CTA/CTAH, ~96% IACS) serves lines up to 200 km/h; copper-magnesium (CTM/CTMH) carries the 27-31 kN tensions of high-speed lines; TB/T 2809-2026 adds copper-chromium-zirconium (CTCZ) for the next generation of high-tension catenaries.
The trade-off
Alloying raises strength and softening temperature at some cost in conductivity. Wave propagation speed rises with tension, so high-speed current collection demands tensions pure copper cannot sustain, especially after wear reduces the cross-section. This is the technical background to the removal of pure copper grades in the 2026 edition.