Leeds Approach
I admire your passion for your local airport but you have to be realistic about what is possible, LBIA is hemmed in on all sides by either property, roads or land that isn't practical to build on.
Whilst in theory a tunnel and rail link could be made into the Leeds/Harrogate rail line it just isn't a practical proposition given the costs mainly off the back of the very different elevations, there isn't space at Horsforth rail station to make any meaningful link, its in a cutting and again surrounded by housing. A link to Guiseley would pose similar problems.
LBIA maximum capacity without a full length parallel runway and given the types of aircraft using it is probably no more than 5-6m a year and given the bulk of its traffic is bucket and spades on a weeks vacation, most of whom will get a cab or self drive a train link is a non starter.