I would dissassemble and clean it.
Soaking it in a solution might damage the insulation on the coils, may not clean all the gunk out, and destroy any lubrication. Lots of times, the main culprit in Lionel, AF, Marx, and other 'toy' train loco's is carpet fiber, pet hair and household dust. All the soaking in the world won't get that stuff out.
If you disassemble, you can check the condition of bearing surfaces, brushes and wiring, too.