Look directly to the south. You will see a hill that has an
underpass.
An engine will push a collection of to the top of the hill where
they are uncoupled and, using gravity, will be sorted into
different tracks.
The loud squealing you will hear is the sound of the cars being
slowed down by retarders.
From Wikipedia...
The heart of these yards is the hump: a lead track on a hill
(hump) over which the cars are pushed by the engine. Single cars,
or some coupled cars in a block, are uncoupled just before or at
the crest of the hump and roll by gravity into their destination
tracks in the classification bowl (the tracks where the cars are
sorted).
The speed of the cars rolling down from the hump into the
classification bowl must be regulated because of the different
natural speed of the wagons (full or empty, heavy or light freight,
number of axles), the different filling of the tracks (whether
there are presently few or many cars on it) and different weather
conditions (temperature, wind speed and direction). As concerns
speed regulation there are two types of hump yards: without or with
mechanisation by retarders. In the old non-retarder yards braking
was usually done in Europe by railroaders who lay skates onto the
tracks, or in the USA by riders on the cars. In the modern retarder
yards this work is done by mechanized "rail brakes" called
retarders.
The Clearing Yard, located on the boundary between Chicago and
Bedford Park, Illinois, just south of Chicago Midway International
Airport, is one of the largest hump classification facilities in
the United States. Some 5.5 miles in length and covering 786 acres
(3.2 km²), the yard supports more than 250 miles (400 km) of track.
It has six main subdivisions: arrival, classification, and
departure yards, in both eastbound and westbound directions.
At the heart of the yard is the wicket-shaped tower which
straddles the hump and from which are controlled the switches and
retarders of both east- and westbound classification yards to
either side of it. Using computer controls, the hump tower
efficiently dispatches more than 8,400 rail cars per day. Operating
around the clock, employees are able to classify between 40 and 50
miles of consists daily.