I should list the steps to find these add-ons. Start by searching reputable communities. Then maybe some specific websites. Also, mention checking for Open BLS files since Open Rails is related but might share some content. But the user asked specifically about MSTS, so maybe stick to that unless Open BLS is a good source too. Wait, the user wants free add-ons for MSTS, so maybe separate sections for locomotives, wagons, routes, and scenery.