Took my new hk55 for a test spin today and made a couple hundred cuts installing bevel siding. Love the ease switching between 90, 45 and compound cuts. What I don't get is the slop on the rail. When the saw is at 45 deg it is hard to cut one handed and wants to tilt off of the rail. Is this common? Also, the rail binds in the saw and does not snap back to the parking area, like commonly seen in the advertising. Did I just get a bad rail, or saw base?

Did you fiddle with the play adjustments in the saw base?   

Try that if not, maybe some paste wax.

I did mess with the adjusters. But according to @festoolsedge the should be complexity slacked off. Which makes for freer movement but lots of play.

Great saw, but yes adjustment is more fidly  than with TS55. I have two HK rails, one is superb, the other - it's like you describe. As for tilt tendencies at 45 degrees, try holding the saw base on the outside edge, the edge opposite the blade. This however means the workpiece should be clamped.

...Also, the rail binds in the saw and does not snap back to the parking area, like commonly seen in the advertising. Did I just get a bad rail, or saw base?

For the rail binding, you need to adjust the cams until it slides freely but there's no slop.There's a video for the TS (same principle) here:

Or did you already do this and still have problems?

So, I brought the saw back to the store where I picked it up. They assumed it was supposed to bind like that..... There's does too. They also adjust the gibs up for precision. We plaYed around with my saw and track and a couple of other saws and tracks and realized it's the riving knife rubbing against the splinter guard and lifting the saw up 3-5mm that's causing the binding. I not my saw, as I need it this week. The store is going to contact their Festool rep to see what can be done....

Seems to be a design flaw. Rail is attached with a centrally located t-track while pendulum cover is pressing the edge of the track down skewing it. On Mafell cross-cut rail slides over the saw base hugging the edges.

Happened too bump into two Festool reps when I went to my local supplier today. The store had already mentioned my issue to them, so it was complete statutory serendipity....  They looked over my saw and track, tweaked the gibs and looked it over forward and backward. One of them thought it should just be exchanged. The other asked the store owner for some lube. He produced a spray can of "deep creep" lube he swears by. I was hesitant as I prefer dynaglide, but don't have any at the moment. But I caved. A short squirt and a wipe down and the track flew back every time. The rep, Gil, focused the spray only on the t-track. After the lube I was able to snug up the gibs a bit to reduce slop. So happy to have this awesome, versatile saw running as it should!