They could follow the old Ewa rail route, which would service the areas that needed it the most Waianae, Ewa, Waipahu, etc. They don't have to use the same rail, just follow the same route, it is there and has been there for a hundred plus years...
Right now it would service the areas that are the easiest to get onto the freeway. What good is that, and does that make any kind of sense??
But forget all of that, because a rail does not make any kind of sense at all.
No one will go through the hassle of driving or busing to a rail station, transfer to a rail, then bus to the destination. Why go through that? Just jump straight on a bus.