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The AirTrain might be another option. They pick you up from your front door and drop you off at the station. The train then drops you off right at the airport either domestic or international and it's a very short and easy walk from the platform to check in
Sorry to be negative but I wouldn't recommend that. It just takes one idiot along the line to do something stupid on the railway line which will cause bus substitution. Which could mean the OP's parents having to drag their cases off the train, up and down and around a railway station and carpark twice - and possibly miss the flight if QR/TL can't be bothered finding a direct bus or taxi for them.
I wouldn't recommend it either, it takes way too long, you're forced to endure the same announcements the whole time and it goes through some fairly seedy suburbs...I do my best to avoid it.
It has its place but dont think its a viable option for these pax.
TG