From a couple: Without regard to any dancers they may be facing (i.e., without doing a Pass Thru), the dancer on the right crosses to the left in front while the dancer on the left crosses to the right in back. Finishes with two dancers back to back (Equivalent to Partner Tag).
BACKGROUND NOTE: The distinction noted here between Crosstrail Thru and Crosstrail is not universally accepted by any means. Merely by saying "Crosstrail" to facing couples, you will probably see the whole floor do a Pass Thru first and then everybody will change sides and turn back on partners looking for corner. It takes careful and deliberate training of dancers in the distinction between Crosstrail Thru and Crosstrail before the distinction is of any practical use.
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.