If it was posssible to build a railway link to Bristol Airport - given its elevation at above 600 feet AMSL it would be a mammoth civil engineering task to link the main line several hundred feet below but within about three miles distance. If it was viable there would then be the need to find rolling stock and pathways on the main line to reach Temple Meads. At best the frequency would be no more than half-hourly.
I still think there must be solutions to the engineering challenges, e.g. build the airport station much lower and have escalators/lifts going up to the airport terminal.
According to the latest draft of the Joint Local Transport Plan (JLTP4) a heavy rail "Bristol Airport Rail Link" is also part of the Bristol South West Economic Link (BSWEL) scheme which is still in early development stage. Not just one, but two phases:
• Package 6: Rail options: Bristol Airport Rail Link Phase One: Bristol Airport to Bristol Temple Meads
• Package 7: Rail options: Bristol Airport Rail Link Phase Two: Bristol Airport to Bristol Temple Meads, Severn Beach/Bath Spa, Bristol Airport to Weston-super-Mare/Taunton
If the plan was simply to come up with an updated feasibility study for a heavy rail link in order to be able to exclude it based on cost or cost-benefit ratio and bolster alternative schemes like the fabled mass-transit scheme to the airport, then why even consider two phases?