at the time the colonial government decided to implement a national railway gauge, they made exceptions for some existing railway systems that had already been laid using different gauges, generally on the proviso that any new track was to the national gauge. one such system was the railways of the canterbury province, which were brought under the jurisdiction of a special act of parliament, the canterbury gauge act. this act made provision for the regauging of existing lines by laying a third rail between the two existing rails to allow for the use of narrow gauge rolling stock on the same track. rather than incur the inconvenience of this approach, the provincial government decided to lay a new narrow gauge line beside the existing broad gauge line from addington to lyttelton. the narrow gauge line reached christchurch on 7 march 1876 and lyttelton 34 days later.