The central section from Te Rapa near Hamilton to Palmerston North was electrified at 25 kV AC between 1984 and 1988 as part of the Think Big government energy program. Some tunnels were opened out or bypassed by deviations while in others clearances were increased, and curves eased. The section between Ohakune and Horopito was realigned with three viaducts replaced to handle higher loads and speeds. The most notable bridge replaced was the curved metal viaduct at Hapuawhenua by a modern concrete structure, though the original has been restored as a tourist attraction.