inside rail slides

You want the sliding rail to be on the inside of the curve. That way it will be longer than the fixed rail, and you can trim the sliding rail to fit.

 If you do it the other way the sliding rail on the outside will be too short. I guess you could trim the fixed rail and remove ties to match, but that is more effort. 

For S curves, you put the sliding rail on whichever orientation that results in it being too long, if that is possible.  If the esses are about equal, the rails will end up about the same either way. 

And a suggestion

If you are using flex track around a curve large enough that one piece isn't enough you should solder the joints on the pieces before curving them and installing them.  This will prevent kinks at the joints that result from joints that are just connected with rail joiners without solder.

Shinohara

If you are using Walthers 83 it doesn't matter. Both rails slide and the ties are connected to each other on alternating sides. Results in minimum waste of usable length when making curves.

Ditto Rick's suggestion. For smaller radius curves make sure to remove sufficient number of ties so the joiner doesn't jamb them up on the inside rail.

These curves were made from flex track soldered together while straight and then bent to desired radius. Kink free.

 "If you do it the other way

If one rail is too short just slide the rail from the next section out till they meet( staggered rail joints).  To avoid removing a tie under the joint one can grind off the top of the tie where the joiners rest( or melt it lower with a soldering iron with some practice). ......DaveB 

Replacing ties at joiners

You still have to first remove the tie(s). Can't grind the top of a tie while it is on the rail. Simple enough to place ties back under the joint when everything is finished. The joiners are very thin stock. Only the slightest depression needs to be ground/cut/melted in the tie area under the joiner. Slide the replacement tie under the rails beyond the joiner and then slide the tie laterally along the rails to fit it under the joiner.

"You still have to first

     It depends where the tie falls in relation to the rail end. If the tie is just beyond the staggered rail joint then it can be ground lower, or cut lower with an exacto knife. I usually just melt the joiner into the tie whereever it falls after cutting off the molded on spike heads. I also cut the rail joiners in half before using them as they are longer than they need to be....DaveB