The "trailer bike" rear-half-bike systems (like my Burley Kazoo) have much, much less drag than the sit-inside two-wheel trailers (like my D'Lite). On the Kazoo, my son is sitting directly behind me, in the draft zone, with minimal added wind resistance. I keep the tire pumped up to maximum to reduce rolling resistance. And, he helps pedal! It's a lot like a tandem in terms of fast cruising speed on level terrain, although braking and steering stability are not as good as with a tandem. You won't accelerate as fast, and you'll want to stay under 25 on downhills, but on a rail trail this shouldn't matter.

The narrower width of the Kazoo also makes passing on greenway/rail trails easier than with the D'Lite.