No passenger, mail or baggage car, except when in mixed trains composed of passenger and freight cars, shall be heated by common stoves or by any method of heating that has not been approved in writing by the department of transportation, or that is not permitted by them in the making of experiments.
RSA 247:12
1887, 95:1. PS 160:13. PL 251 :23. RL 301:23. 1951, 203:60 par. 12. RSA 377:12. 1985, 402:6, I(e)(7).