Short answer: A section insulator electrically segments one supply arm (station throats, loading sidings, depots) - both ends are the same phase. A neutral section insulator separates two different phases at the phase break; it withstands the inter-phase voltage and trains coast through with the main breaker open.
Structure and passage
Neutral section insulators carry more insulating elements with larger air clearances and creepage. Automatic phase transition uses track magnets and onboard equipment: open breaker before the neutral zone, coast through, reclose after. Speed ratings for section insulators include 160 km/h and 200 km/h classes - match the line speed when selecting.