Railway-duty design
This breaker carries a railway-specific designation, meaning it's built to handle the vibration, temperature swings, and DC supply transients you get under the carriage or in a locomotive cabinet. Vibration resistance per IEC 60068-2-6: ±1 mm at 5–25 Hz, 50 m/s² at 25–150 Hz.
Thermal derating — 63 A is the 35 °C figure
Rated 63 A at 35 °C. At 40 °C it's 58.28 A; at 50 °C, 53.11 A; at 60 °C, 47.38 A (–). If the panel ambient sits at 50 °C, you're sizing for 53 A continuous — not the 63 stamped on the front.