To make them viable though, you need services every 15 mins or so, and the station ideally needs to be withing walking distance of the terminal (or via a frequent tram / people mover). Stations that don't provide an adequate connection to the terminal, like EMP and DTV, don't get used much.
DSA hasn't the PAX to consider such a huge undertaking.
With a major expansion and PAX growth the airport could consider a short extension to the Finningley line, bringing it to within walking distance of the terminal. But the cost would be enormous.