The rehabilitation of the Churchill branch is an interesting and complex engineering problem.
Nature Magazine (27 sept 2007) pp 398-402 had an excellent article on construction of the Quing-Hai railroad, a 416 mile line over 40% permafrost. They used extra-high embankments with frequent transverse ventilation ducts. Thousands of remote temperature sensors were emplaced on both sides of the railbed. It was concluded this was the only method of insuring safety on single track in the middle of nowhere. There was preventative removal of icy masses from the frozen base which was filled with high consistancy soil. Finally active cooling (refrigeration with pumped cold saline) was used on several critical sections.
The Chinese experience is the most recent and well-documented.
Google scholar has a scientific search engine, including material on cold weather permafrost rail construction from the 11th annual cold weather engineering conference.
