I built my first 2" x 4" rail yesterday. It is 53" long so it fits in my trunk and has the narrow side up. On the second attempt I rode it to the end on my new Tuni. In the next 15 minutes, I rode it only about 4 more times. I never even thought I would get up on it at all.
Now, I have been practicing on a longer 3" x 6" and I have had a dickens of a time trying to ride that. But, then I get on this 2 x 4 and ride it out. I am thinking, “Hey, this should be harder, not easier”.
Go Figure. 
I should mention I am mounting from the ground and not hopping on and riding.
Anyone have a similar experience with something that looks harder, but was easier? or any theories why?
I think that part of why the rail is so easy for you is that for something 53" long, you don’t have to be in balance in order to ride the whole way. Try riding the 2x4 which then connects to the 3x6. This is the real test of whether you’re in balance.
If you were saying it were a 1x4 I wouldn’t be surprised that it’s easier, since at that point the tire literally wraps around the beam and keeps you going straight, but that’s not the case with a 2x4. Other than the above paragraph, I’m at a loss to explain it, although next time I do reccomend making longer balance beams. I have a 5’ one which really isn’t long enough until I put it end to end with my 8’ one. Then it’s barely long enough. I think a balance beam is really only good for practicing if you can get 2 or more complete pedal revolutions on it before reaching the end.