How do you choose your locomotive?

A regional train carrying a few hundred passengers and a train transporting thousands of tonnes of merchandise
do not have the same requirements in terms of traction power in order to move forward.
For each type of rail transport, therefore, there is a different type of traction, i.e. a different locomotive!
In general, the more axles there are on the locomotive, the more weight
it can tow:

How do you choose your locomotive?