Mont. Code Ann. § 50-74-104

Locomotives, commonly known as dinkey engines, used in operating logging or mining railroads or any similar work where such locomotives are owned, leased, or operated by any individual, company, or corporation and are used in the business of such individual, company, or corporation and not for general commercial purposes shall be classed as traction engines and be subject to inspection as are other traction engines, and the persons operating or firing such dinkey locomotives shall be required to hold traction licenses.