With the new coalition Government holding the reins there seems to be a widespread belief that heavy trucks will be tossed off our roads and freight moved by rail instead.

Unfortunately that's almost certainly not going to happen, for a variety of reasons.

The most compelling is the expected growth in freight. A national freight demand study reckons total freight moved in 2017 was about 260 million tonne, growing to 354 million tonne by 2037. About 7 per cent of that is now moved by rail. If rail tripled its share of freight by 2037 there would still be 20 million tonne of the growth to be handled by trucks.

That's 8 per cent more than carted by trucks this year, meaning more, or larger, trucks on the road.

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Almost everyone in the freight industry agrees that coastal shipping, rail and road transport all have roles to play in freight movement. Freight forwarders such as Mainfreight are already among the highest users of rail.

However, the very nature of freight movements around New Zealand make it almost impossible for rail to double, let alone triple, its share of the task.

In the early colonial days most freight was carted on river boats and coastal ships. After the first railways were built in 1863 it soon became obvious trains could move freight to more places than boats could, without being hampered by wind and tide. Now coastal shipping moves about 2 per cent of freight.

After World War I trucks became increasingly efficient at moving freight and the Government, which of course owned the railways, began in 1931 to introduce measures to protect rail freight, culminating in Bob Semple's 1936 ban on trucks carting more than 30 miles (50km). Protection of rail was scaled back in 1977 and abolished in 1983.

One of the hurdles to a rail comeback is that heavy trucks – over 3500kg loaded weight – have become increasingly specialised. There is little opportunity for trains to replace concrete agitator trucks, for example.

Although there is a very efficient railway line from the gigantic Kaingaroa Forest to the Port of Tauranga, there's probably no other log cartage operation with sufficient scale to replace trucks. Livestock trucks, fertiliser trucks, fuel tankers, milk tankers and many other trucks operate miles from any railway line.

Then there are delivery trucks, from the big tractor/semi-trailers delivering to supermarkets to little four-wheelers dropping off a new fridge. Or trucks taking supplies to rural centres too small for a railway line to be economic.

In fact of the 132,000 heavy trucks on the roads, only 23,000 are operated by the road freight industry. Put another way, just 19 per cent of freight is general freight and when distance is taken into account, just 8 per cent of the annual tonne/km carted is general freight. The biggest proportion, 29 per cent, is manufactured and retail goods, followed closely by logs and dairy.

Rail and coastal freight are best at carting bulk freight that is not time-sensitive over long distances. New Zealand's small, widely dispersed population restricts opportunities for that. I once sat at a level crossing in North Carolina as an extremely long train passed, carting only orange juice from Florida, probably to New York. That one train probably contained more than New Zealand's annual consumption of orange juice.

There are other handicaps, too. For example, the most efficient freight trains in the United States cart shipping containers two high. New Zealand's small rail tunnels don't allow that.

Another issue is that while modern freight handling systems have greatly reduced damage and loss of goods, most problems arise during loading and unloading. Most rail freight has to be carted to and from the train by trucks, which means three times as many handling events compared with a single truck journey.

Just as in the 1880s trains took over from boats because they could cart freight to more places more quickly, trucks have since overtaken rail for the same reason.

The combination of an expanding freight task and the difficulties rail faces in increasing its share mean we're more likely to see more trucks on our roads than fewer.

• Jon Addison is a retired journalist who specialised in commercial vehicles and road transport for 30 years.