Huntly rail bridge bombing

The Huntly rail bridge bombing occurred on the Glen Afton (or Awaroa) Branch, near Huntly, New Zealand on 30 April 1951. The bombing took place amid the 1951 New Zealand waterfront dispute, an industrial dispute over the working conditions and wages of dockworkers. Characterised by the-then Prime Minister Sidney Holland as an act of terrorism, the bombing caused no casualties but disrupted the supply of coal from nearby coal mines. The perpetrators remain unknown.
Bombing and result.
In 1951, during the New Zealand waterfront dispute and strike, an explosion destroyed the Huntly rail bridge. The bridge linked four open-cast mines and several pits in the Waikato coalfields with the Huntly township and the railway line, so its demolition was intended to disrupt coal supplies. 
Train drivers were warned beforehand, so there were zero injuries or casualties. The attacks are thought to have been carried out by coal miners operating without the knowledge or support of the trade unions involved in the industrial actions.
Possibly due to reporting restrictions at the time, there have been conflicting reports. Two Australian reports say that a passenger train crossed the bridge about four hours after the explosion. They also say the bridge wasn't destroyed, but that piles and stringers were damaged by six dynamite shots, which failed to destroy it because they were against the grain of the wood. It has been speculated that the charges were carefully misplaced with the intention only of warning open-cast miners who were working.
Reaction.
Prime Minister Sidney Holland denounced the bombing, calling it an "infamous act of terrorism". Holland announced an investigation, but the identities of the perpetrators were never discovered. However, it has been argued that the act was one of sabotage, rather than terrorism. The probable target was property and the intention was the disruption of supplies, rather than achieving a political aim through terror.