This should be a good thread Charlie.  For me it was always Missouri Pacific.  I was born in Labadie, Missouri, and the Rock Island was the "show" so to speak since it ran a block from my house.  I actually remember the house we lived in faced the Rock's line through town, and as a little kid (prior to first grade) I used to push blocks around on the sill of our screened-in porch. mimicking the local's switching moves (yes in the early 1950s the Rock actually had business in Labadie!).  Got into big trouble with my parents when they had a First Communion party for me and I convinced an elderly aunt to walk with me up to the tracks to see what a turnout looked like.  Everyone at the party wondered where we were. 
Anyway, this is MoPac, right?  The MoPac's main line ran to the south of Labadie, crossing Kiefer Creek on a high bridge just before entering the west tunnel running under Highway T.  I could see the trains running up there, and although I remember seeing the Eagles fly past, I liked freights because they were longer and you could watch them for awhile. In 1960s we moved to Washington when my Dad opened an appliance store.  We lived at the north end of MacArthur Street and the MoPac main was just down the hill, about a five minute walk through the woods.  I was seven, and pretty restricted to going over to the top of the hill and watch the train go by.  When I turned eight, my paternal grandfather decided it was time for a train trip, and my brother and I met him in Washington (he had come out on the train from St. Louis or Kirkwood, and we went to Jefferson City. I was hooked for life. I loved the Eagle colors, the smell of creosote and diesel exhaust, the speed of the train. I expect it was the MRE since I pretty well remember the observation and speedometer, at least I think the obs was still running in 1961. There was definitely a speedometer.
My dad got occasional shipments of appliances by rail and they came in Eagle merchandise cars.  I remember helping him unload the Frigidaire's and Hotpoint's.  He'd advertise them as "carload sales."
My grade school friends and I spent a lot of time down at the tracks.  A busy double-tracked railroad and a mighty river were candy to young boys.  The tracks were a short cut to downtown Washington.  Need to keep in mind too that Washington in the 1960s was one of many small town where a Mom could be quite comfortable turning her kids lose at 8AM on a summer day and not worry about them unless they didn't show up for supper at 6PM. We camped out a lot between the railroad and river.
Much later in life I learned that the valley to the west of our house (between MacArthur and Burnside) was once a wye used to turn locomotives used as helpers over Gray Summit after the railroad eliminated the turntable and roundhouse in Washington in the 1920s.  Never saw any evidence of the wye, even though I probably covered every square inch of that valley. Anyone out there have any information on this?
Had an American Flyer layout in the basement,and of course the Missouri Pacific freight and passenger trains they offered.
Anyway, I never worked for the railroad, it was just a common part of life.  I kind of lost interest in railroads and modeling during adolescence, but took it up  again during college.  Actually I have to recognize the Union Pacific as the catalyst that started it all over again.  It was hard not to notice the UP in Laramie in the 1970s.  But, when I did get back into railroad history and modeling, it was the MoPac I turned to after a brief, but interesting excursion into the peculiarities of the Nickel Plate Road (John Rehor's book on the NKP was awe inspiring).  I have been fortunate since that time to watch interest and appreciation of the MoPac grow over the last 30 years,  Now I even have a chance to reproduce the railroad in miniature, but curiously not in Washington or the surrounding areas.  To big of a railroad for the space, so, thanks to Joe Collias' influence, the southern Illinois coal fields, and still a fascinating railroad.