Let's do it properly and go under Bramble Bay from Sandgate or Deagon
The 2002 study that decided that a line linking Kippa Ring to Redcliffe wasn't warranted was an extraordinarily foolish decision, made just before petrol prices went through the roof. Since then, public transport use has been rising every year.
Derwan's right - the majority of people getting off the train at Sandgate and at North Boondall are clutching books from the Redcliffe Library! Of course they should have added a rail-line to the new Houghton bridge. It would alleviate parking problems, traffic problems, etc. etc.
However I think that the rail corridor between Redcliffe and Kippa Ring has now become a part of the problem, not part of the solution. Putting a Redcliffe spur onto the already overloaded Caboolture line would be very costly, because so much duplication of track (and platforms) would be required to take the extra traffic. It would also increase the number of trains per hour through several level crossings. It all becomes too difficult, so nobody does anything (except promise to do something during elections).
It would make a lot of sense to look at running a line (above the waterline!) from Deagon or North Boondall instead. There's already a lot of land on that route in the public domain - beside the Deagon Deviation and the Gateway Motorway, for instance, and the underutilized Deagon racetrack. Some of it is wetlands, so environmentally sensitive and there would be engineering issues. But for a spur line, a double track built up on whales tails (like the airport route, also across wetland) would work. Even Bramble Bay isn't insuperable - especially as a railway line could go upstream from the Houghton Highway, where Hayes inlet is narrower.
Laughably, part of the cost of putting in the new Houghton Highway was that it has been built to specifications allowing for the sea level to rise because of global warming. Replace all those cars travelling to Sandgate station with a decent public transport system from the peninsula, and they might be able to build the railway bridge lower!