That's what I suggested to merge UPX and RER. The same train can be used for both UPX and for RER routes.
I have a thread
The Great Platform Height Debate.
GO has a pictogram of the KISS potentially being used for RER:
- Order mainly low door option for core electrified GO routes (serves GO platforms)
- Order at least 30-40 coaches with dual level doors (serves GO low plat, GO high plat & UPX platforms) with baggage racks.
Then plenty of UPX spares, which can also run on the rest of the RER network when not used for UPX.
Fleet commonalty opportunity?
This proposed GO EMU has a dual height door option too, that can serve UPX platforms.
Basically, the Stadler KISS train, with the optional high door option. Get some with, get some without, and gain fleet commonalty.
And you can use one 4-coach trainset for UPX. (station modifications needed to lengthen to 4 coaches instead of 3 coach, but that's just details). Midplatform baggage racks and accessible seating area on the midlevel areas, long-distance non-Airport commuters encouraged to sit on the high deck.
BiLevels aren't a service speed problem if you've got the major boarding delays only at the terminuses (e.g. at Union and at airport). BiLevels are a problem with boarding speed with many middle-of-route major stations (e.g. Sydney) that slow down the train to being unable to do 2-minute headways. That's why Paris RER double deckers are often not a problem in Paris, because they seem to use them for routes where the major boarding surges are generally at a downtown terminus with their own dedicated platforms. Just like UPX! So I think BiLevels are okay for UPX given this consideration, BiLevel type trains will not slow down UPX because of this and 2-minute headways aren't needed anyway for UPX -- you can still easily upgrade to do 10-minute headways down the road -- they dwell the UPX train at Union for almost 10 minutes often! So a 5 minute dwell is still generous for a BiLevel type train -- and even a 2-minute to 3-minute dwell is possible if service frequencies ever need to be increased to a 7.5min interval.