Steam Engines, beloved in song and story

Tootle, The Little Engine That Could, Tom Thumb, Hogwarts Express, Polar Express, Oriente Express, Broadway Limited, Thomas,we still love them.  Diesel locomotives replaced the steamers by 1957 or so but never got into literature or the movies much. Among railroad people steam locomotives were always described by the number of the locomotive's wheels, pilot wheels, driving wheels, trailing wheels.  The standard 19th century steamer was a 4-4-0, four pilot wheels, four driving wheels, no trailing wheels. 
   The earliest steam engines, Tom Thumb is still on display at the Baltimore and Ohio museum.  The design is straightforward, firebox on the bottom, a fire tube boiler mounted  atop the firebox and a stack on top of the boiler.  Flames rose up thru the firetubes, boiled the water, and rose up the stack creating draft to keep the wood fire burning brightly.  Just four driving wheels.  Tom Thumb never went fast enough to need the steadying effect of pilot wheels.  This design was successful and quite a few were built.  But the design does not scale well.  A bigger locomotive needs a bigger taller boiler and the taller boiler won't fit under bridges. 
   New design, that lasted until the end of steam, laid the firetube boiler on it's side, placed the firebox at the rear, where the fireman could reach into the tender for wood, or later coal, and the stack at the front.  Flames from the firebox were led forward thru the firetubes and then up the stack.  Waste steam from the cylinders was vented up the stack to increase the draft and creating that distinctive choo-choo sound. This arrangement needed a pilot truck the carry the weight of cylinders, stack , and the front half of the boiler. 
   In my childhood all small boys knew that you could tell a passenger locomotive from a freight locomotive by looking at the number of pilot wheels.  Freight ran fairly slowly, say 30 mph and a two wheel pilot truck was enough to steady them.  Passenger trains reached 100 mph by 1900 and needed much more weight on the pilot truck to lead the locomotive into switches and curves.  The extra weight needed four wheels to support it. 
    The older smaller 19th century engines located the firebox just over the rear set of drivers.  This worked, but it limited the width of the firebox to 4 foot, eight and a half inches, the track gauge.  Larger locomotives built after 1900 moved the firebox clean aft of the drive wheels and widened it out to 10 feet, the widest it could be without hitting station platforms.  And a pair, sometime two pair of trailing wheels were added under the firebox.