MLH from Overland Park KS on 6/2/2006 8:47:53 AM:
Check out this link to an article that addresses tires for tandems in particular (although they mostly discuss 700c):
http://www.precisiontandems.com/arttiresbymark.htm
The last section on crushed limestone is what you are dealing with on the Katy. Although I have no personal experience with tandems on rail trails, I have used Mavic 559 X 17 rims on my hardtail MTB for hundreds on miles therein. Have had the best luck with no punctures using Continental TravelContact 26 X 1.75 (559 X 47). You can use this on pavement also but not for mud.
http://www.conti-online.com/generator/www/de/en/continental/bicycle/themes/tires/city/travelcontact/travelcontact_en.html
Also, check out the Schwalbe Marathon XR. There are several dealers in the Seattle area found in the website dealer locator. Ask them about the proper width (and psi) for your load weight.
http://schwalbetires.com/node/166/ok
No experience with the Avocet Cross II in the article above but you would want the protective shield version.
Also, the first two above come in folding versions, nice to pack a spare for long trips.