THE LEARNING STATION
Look at the wheels on Locomotive 3768! Did you notice that the wheels are not all the same
size? This locomotive has an arrangement of wheels referred to as a 4-8-4 wheel arrangement
commonly called “Northern” after the railway that introduced this type of arrangement (Northern
Pacific Railway).
Northern-type locomotives have
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Four leading wheels proving stability, balance as well as guidance around curves and switches
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Eight driving wheels on four axles, coupled and powered by the locomotive’s pistons
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Four trailing wheels supporting the cab and firebox
How fast a train is propelled forward is directly tied to the size of the driving wheels and how
many times it rotates per minute. Larger diameter driving wheels travel a farther distance per
rotation. A trade-off of larger driving wheels is that they offer less stability, especially at higher
speeds. A common solution to this lack of stability is the addition of non-driving wheels, but
these reduce the portion of the locomotive’s total weight supported by the driving wheels
resulting in lower traction. This Northern-type locomotive with its 4-8-4 wheel arrangement and
80-inch diameter driving wheels was best suited for express passenger and fast-freight service.
Due to focus on freight, the primary revenue source for railroads, and the limitations the
Northern-type steam locomotive had with its pulling power, these models of high-powered steam
locomotives were retired and replaced by diesel-electric locomotives.
Learning in Motion
Compare the sizes of the various wheels at this station to the 3768 Locomotive driving wheels
Roll the various wheels along the ground ruler and notice how far each wheel goes in one
rotation.
You’ve just measured the circumference of a wheel! Circumference is the perimeter of a circle,
or in other words, the length of the outside edge of a circle, such as a wheel. Find the marked circumference of the 3768 driving wheels along the ground ruler, how does this
compare to the wheel you just rolled?
See how many times your wheel has to roll in order to roll the same distance the 3768 driving
wheels roll in just one rotation.
Walk the distance marked for the driving wheels, imagine doing that 294 times in a single
minute! That is how many times these 80-inch wheels rotate to achieve a speed of 70mph.
How Do We Describe Circles?
Driving Wheel Rotations at Various Speeds for Locomotive 3768
