Is your loco sluggish? It could be your commutator and/or brushes! Here's how I remove and replace the brushes for cleaning!
Here's the brushes of a postwar Alco loco. (We're not going to go into the brushes of the tender unless someone asks, as that's a different kettle of fish!)
Note how the springs are aligned with the slots in the brushes:
Undo the two screws and pull the top off. Note the brushes in their holders
Gently clean the holders with a Q-tip. If the commutator is dirty, clean with a track eraser or wire brush. Don't use sandpaper! And be sure to clean between the segments!
Reinstall the top with the commutator brushes still out.
Using tweezers, GENTLY tug the wire aside and reinstall the brush.
Once the brush is installed, align the slot with the wire.
Repeat with the other brush, put your loco on the rails and watch it zoom!