Dave
It's a TOY, A child's PLAYTHING!!! (Woody from Toy Story)
If you've cleaned the commutator and the brushes are not worn out, consider an armature fault. One or more armature windings may be open or have shorted turns. If you have an ohmmeter, you may be able to compare the resistances between commutator segments in pairs to see whether they are consistent. This is three measurements if you have 3 segments, as I imagine you might. If there are more segments, look only at resistances between the most widely-spaced pairs of segments.
Bob Nelson
Make sure there is no crud in between the segment faces shorting the segments; clean with a toothpck.
Jim
Before you go blaming the armature make sure that you have good spring tension on the brushes, proper brush tension is very important on the older motors, and the brush surface is flat and clean. Also check the wheels on the engine and tender for dirt buildup and clean as needed, may have to remove the wheelsets on the tender(be sure to put the wheels back correctly as one is plastic and one is metal) to clean the contact points underneath, usually copper pieces of metal and can be cleaned with a stiff pencil eraser. Lionelsoni may be correct but very seldom do armatures wear out, ckeck the electrical connections first!
Lee F.
"If you've cleaned the commutator and the brushes are not worn out, consider..."
And "sparks from the mid engine area" don't suggest to me a problem under the tender.
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