This article is from the Model Trains FAQ, by Christopher D Coleman with numerous contributions by others.
Remove the shell and milk can ramp cover. Early shells are removed by
first removing two wire clips under the car which pass through the
frame and shell on both sides. Later cars have screws on either end of
the car. Clean every trace of dirt, oil and grease from inside the
milk can ramp. This will usually solve the problem. Also make sure
your platform is set to the correct height for O or O-27 track using
the appropriate slots for the metal platform.
 
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