The Main North Line took over half a century to build, and passengers using the northern portion in Marlborough were primarily catered for with mixed trains, while the southern section was the route of the Culverden Express. As of the mid-1920s, the Culverden Express began to terminate at the coastal Parnassus terminus rather than the inland Culverden terminus, and this was the forerunner to the Picton Express. The route north of Parnassus was completed as the Main North Line on 15 December 1945, and from this date, the Picton Express was introduced, operating the length of the line.
The Picton Express began life as a daily service, offering a swifter connection between Canterbury and Marlborough than had previously been available. However, after operating for only a month, coal shortages in January 1946 meant that it was cut to thrice weekly, operating on Mondays, Wednesdays, and Fridays. It operated to this schedule for the rest of its existence, except for when extra trains had to be run at peak holiday seasons such as Christmas and Easter. In 1950, the northbound journey took 8 hours and 15 minutes, departing Christchurch at 8:25am and arriving in Picton at 4:40pm. The southbound journey was slightly longer, taking 8 hours and 22 minutes; its Picton departure was at 9:45am and it arrived in Christchurch at 6:07pm. The two services passed each other north of Kaikoura, and in Blenheim, passengers could make road connections to Nelson. As the Railways Department did not operate dining cars at any time during the Picton Express's life, stops were made at Waipara and Kaikoura for passengers to purchase refreshments. The train was always hauled by steam locomotives, typically of the AB class.
As the 1950s began, competition increased from airlines, buses, and private cars. Passenger numbers began to dwindle, especially during off-peak periods, and the Railways Department began to investigate railcar alternatives that would be better suited to the service. In February 1956, just over ten years after the Picton Express began operating, it was replaced by RM class 88 seater railcars. When the railcars wore out in the 1970s, they were replaced by a carriage train that was later rebranded as the TranzCoastal. Before the rebranding, this carriage train was sometimes also referred to as the Picton Express.