I've had a T-900 for years and it is great for touring with kids. It has a lot of adjustability for seat height and handle-bar reach, so it is easy to adjust for different size kids riding day-to day. Works will for small-to- medium adult on back too. Only modifications I made was to change to clipless padals for captain, and added toe-clips for stoker, and put on 26 x 1.50 smooth tires for pavement riding.
There is a unique TREK hitch option built into the frame, under rear seat (pull out round rubber cap in center of frame), that you might try to use to pull the tag-a-long. I think Trek stopped making the hitch pins, but you might still find them at Trek dealers. It would make a very stable hitch for the tag-a-long.
Last edited by Ben There; 08-01-10 at 01:32 PM.