P.S.
What I have working so far. Not sure it's optimal but to go to the first record, the following syntax appears to work.
xdXDocument:GetDOM(''Trips'')/dfs:myFields/dfs:dataFields/d:Trips[preceding::node()]/@TripID
To go to the last, the following works:
xdXDocument:GetDOM(''Trips'')/dfs:myFields/dfs:dataFields/d:Trips[last()]/@TripID
Ok, ...