Provision of clearance for high-voltage overhead lines under tunnels and bridges is a major part of the cost of a new railway-electrification scheme. These costs have been reduced over the years by the use of glass-fibre insulators and small working clearances. The paper describes a development in glass-fibre-insulator design which allows much smaller clearances (100 mm) to be used at 25 kV AC. It also outlines the failure mechanism peculiar to these insulators caused by iron-brake-shoe dust and describes an accelerated iron-oxide/fog-chamber test used to evaluate new coating materials for glass-fibre insulators.