rail lines are also more expensive than roads to maintain

That's because they transport more material than roads.

The NZ government did a thought experiment where they shifted all rail to road, and the maintenance costs would increase by $105 million.

Keep in mind the rail system in NZ is underdeveloped.

Source: https://www.kiwirail.co.nz/assets/Uploads/documents/2021-Value-of-Rail-report.pdf

If you want to shift the most materials from one place to another at the cheapest rate, you would use rail.

the cost of moving your items depends entirely on how many items you move—sometimes roads will be cheaper, and sometimes rails will be cheaper

Do you mean cost to the end consumer or actual expenditure? Are you including CAPEX? What are you actually talking about?