Ductile iron pipe is "pre-insulated" for many applications (also including hydronic services etc.), with various insulations and jackets applied in somewhat controlled Shop circumstances by many applicators. It is probably also at times field-insulated for various reasons by mechanical contractors etc. I believe pre-insulated pipe vendors in the USA include Thermacor, Perma-Pipe, Urecon and probably others. You could find out more about insulation and other design aspects of such specialty systems with a search involving such words/firms, and/or also talking with contractors, Engineers, Owners etc. experienced in such pipes/systems. I would only add that if there can be low/no flow in the exposed line during periods of extreme cold, it might also be good practice to equip the crossing with a means of draining (of course prior to/in the event of such flow stoppages) and/or with a means (e.g. heat tape etc. talked about by Mr. Bimr) to proactively prevent the contents from freezing.