I'm pretty sure it's railroads, because there's not that many highways in Siberia, the rivers would be frozen over, and there's not a ton of people in Siberia. I'm sorry if I'm wrong.
In Siberia climate is harsh and has a long winter of minus 25 degrees celsius temperature and 70% of the oil fields are developed by the Russian and most of the roads become inaccessible in the owners thus railroads have used a means to carry the oil and the pipelines get jammed in severe winters months.
