If other trains run fine on the same transformer voltage may or may not be an issue. However it would be worth checking.
Check all wire connections.
Make sure there is no play in the fingerboards and they are tight and the fingers are making contact with the barrel.
There should be one spacer on the armature worm gear side.
Make sure the commutator is clean.
Check the brushes for excessive wear.
Check the brush springs they should be approximately 1 inch long.
Hope this information helps.