I grew up near a general aviation airport in Spokane, WA and had light-civils and business planes flying over my head from before I could walk or talk! So I guess a fascination with airplanes isn't too surprising. That soon led to airshows with the folks, and by the time I was 8 or so, dad was bringing home models for me. He helped me with the first couple and then I was on my own (to learn such things as PATIENCE the hard way!!
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Also near my house were two railroad lines - one the mainline between Seattle and Chicago - so perhaps a fascination with trains was also somewhat pre-destined! Not long after the model airplanes came in, the model railroading started. The techniques and disciplines learned in each hobby helped the other.
As I grew and progressed through school I found a real love for history, which has fed the fires. I kept up with both hobbies through college... granted, they slowed down quite a bit but when i would come home for semester breaks or for the summer, I would work on a few kits or on my model railroad. As I've moved out on my own, time and space constraints (and frequent moving) have made me more of an armchair model railroader. I still read the magazines and slowly build a collection with the intent of returning to the hobby when conditions are nicer. However, I've stayed very much more active in scale modelling - no need for a layout, just shelf-space for displaying completed works and storing kits waiting to be started!!
Incidentally, my dad who started me in the hobbies did a little of both in his youth too. He was over in my part of the state last week and we visited a couple hobby shops, and I'm getting the feeling he might be re-entering at least one of the hobbies as retirement age approaches for him! Lord knows he'll need to find something or he'll drive mom batty....