From a previous life , it is incredibly difficult to properly evaluate growth or usage on new lines. (or even new services)
Yes there are complex demand models you can build , based on demographics , trip rates , underlying economic and planning assumptions and deliverables. A micro industry here , and well established.
I can say not much more than that, though there have been various "surprises" over the years - mentioned before how the "model" for Llantwit Major to Bridgend was hopelessly inaccurate (estimated 3 passengers a train - reality was about 20+ average , thanks to a cheap zonal fare !) , and "railheading" into various new services can be difficult to estimate.
However a baseload of indiginous population is more than helpful. It helps not to have a population desert for example , likewise densely populated areas with a low propensity to travel can equally challenging.