Correction for Dutch Rail Nut. The only place I've seen wood ties lasting as little as 15 years is Florida. Wood ties in mainline tracks are almost always creosoted hardwoods (usually oak). Of course life depends upon climate and moisture. In the dry, cold West, wood ties commonly last 40 to 50 years. Note that the tie job in the Moffat Tunnel this summer will replace ties originally placed when the tunnel was built in 1930! Of course, they had a roof over them.
In the northeast US, a life of 30 to 35 years is about right. Same in the midwest, but these are only averages. I have some mid-1930s date nails I rescued from ties we were replacing on the Illinois Central in the mid-1970s when I was a track laborer there. That's 40 years.
On the other hand, in 2002 I inspected a ballast-deck trestle in Savannah, GA that CSX was abandoning. The structure was built new in 1982. Essentially 100% of the ties had failed, although no traffic had used the bridge since 1993. That's a 20-year life, max -- that's what a warm, wet climate does for you.
The principal problem with concrete ties is that they're expensive and so are the elastic fastening systems they need. UP has purchased one million plastic/rubber composite ties in the last two years, and has been very happy with their performance. These ties have been in test at Pueblo, CO on the AAR test track, and after one billion gross tons, no failures have been recorded. The composite ties are completely resistant to environmental decay and conventional spikes and plates can be used on them. They're only about 25% more expensive than wood ties (vs. 100% for concrete), and they may be the wave of the future.
