its' good all the way to ellensburg, then gets considerably more sandy and disconnected as it approaches and passes the columbia river. ride it to the columbia is my suggestion.

Falcon Press's Mountain bike Washington book has an excellent, detailed writeup of conditions along the iron horse trail.

it's nice, good riding.

I've ridden the bits to ellensburg on 700x32s and 700x35 road 'slicks'. a schwalbe marathon type of tyre would treat you fine.