Actually, wooden ties are the norm. Concrete ties seem to be installed on high density main lines (the NEC, single track heavy freight traffic, etc) where yearly track projects would slow traffic to a crawl during scheduled wooden tie replacement. Every 2 to 5 years, a tie gang will replace every third or fourth tie along a main line, not every cross tie. If I remember correctly, a wooden tie has a lifespan of 20-25 years depending on how much traffic (tonnage) goes over them. On high density mains with wooden ties, those ties are scheduled for replacement every 15 years. Concrete ties have a lifespan of 50 years, thereby eliminating main line closures that are needed during wooden tie replacement, usually 8 to 10 hours a day, 4 days a week, 10 to 12 weeks long, averaging 2 to 3 miles per day's work. It's a major project for a railroad to install concrete ties, as the entire section being done must have 100% concrete. The expense of the project must be offset by the savings against delays incurred during wooden tie replacement projects. Class 1 railroads carefully consider the need/benefit of installing concrete ties, especially nowadays with fewer mains / alternate routes / detours available when the primary main must be closed for a track project. A concrete tie project will usually coincide with new highway grade crossings, welded rail, ballast cleaning, undercutting, and turnout replacement. A VERY expensive project, as we can see by the few number of mains that have them...