I took the LHT with the tagalong bike. I've not done many rides with the Trek Mountain Train. My dad gave it to us to use with our older kids. But this kid is probably a little too small for it. He rocks back and forth a whole lot and nearly runs me off the road. To be honest, the Xtracycle does the job with a lot more ease. The Mountain Train is less stable and you feel it more in the cockpit. It takes a lot of concentration and control to keep from swaying a foot or two in either direction.
All in all, we weren't out for too long. But we had a blast together. He loves riding on the bike and he loves one on one time. He was singing songs about hanging out with dad, which probably a good sign. I love spending time with my kids. And being on a bike is such a fun way to do it.
Get out there.