If you call CTC on the places that have diesel, like Dallas or Trondheim, there would probably be some weasel excuse why that service isn't *really* an airport train or it's not the *right* sort of airport. The same way they blithely ignore the impact of regional passenger rail and CN/CP freight movements, which will stay dieselised, while assuming maximum operation of the ARL to theoretical limits, irrespective of constraints at any point, almost immediately.
That said, I doubt there's anyone on this board who wouldn't electrify the service, given it would actually be a better transportation solution technically, it's a question of when. The Feds made a balls of this project (when SNC Lavalin and their partners wanted to do it on the cheap with RDCs), sat on it for years and then sloughed it off onto the Province. The Province promised it as part of the Pan Am bid - possibly unwisely, but that's true of the whole Pan Am thing anyway. The chances of building an electric line by 2015 were low and that's before CN and CP came out of the closet in Montreal and said "we don't want electrification on non-dedicated track". Better to have the fight with CN where it needs to be fought - Lakeshore Line - and worry about Georgetown later when the route to Mimico is already electrified, and start putting a dent in the number of taxis hanging around the financial district. Personally I'm not wild about electrifying the ARL until it's a proven financial winner given that the ARL was planned before Porter existed and is unlikely to deal with all the reasons YTZ is pulling in more services.
As for third rail, since the case that was taken in Chicago where in 1977 a drunk guy got electrocuted after trespassing at a level crossing despite plenty of warning signs and what not, I don't see transit agencies building a new non-grade separated service with third rail unless they had no other option (what LIRR and MetroNorth have in their legacy systems is a different matter) because of the likely cost of public liability insurance - especially in Toronto where people have no compunction about trespassing along and across the poorly secured heavy rail alignments where they feel the nearest crossing is "just too far".