It is not the radius, it is the surface of the rails, both inner and top, that are giving you the problems.
First things first, though. The engine specs out? No gauge issues, nothing hindering the trucks from pivoting when the engine is on the rails. No flashing or binding somewhere? If those are all okay, then we must turn to the tracks.
When I introduced my Lionel Challenger, that is when I learned that new engines mean new problems. Since then, I have had to refine my bad spots when a BLI Niagara 4-8-4 came later, then a BLI Pennsy Duplex 4-4-4-4. All three locos humbled me until I gritted my teeth and began to track down the problems. As Chip says, they derail here, but what makes them derail here is something about where they have just been. So look anywhere from the mid drivers and on back to the rear coupler on the tender. Somewhere in there is a bad joint with a sharp edge or there is a dip or a hump...and those two don't have to be at a joint at all. Poorly supported flextrack will sag over a couple of inches. Turnouts can really sag and cause wobbling if they don't float properly.
Use good lighting, use a straightedge, and use a small handmirror to see which axle begins to lift out of the rails first. Then look back to the drivers to see if one or more, on either side, is slightly suspended above the rails. Alternatively, place a smooth straightedge atop the rails in several locations and backlight where the two meet. If you can see widening bands of light between the dark underbody of the straightedge and the rail tops, that is where you must shim or reduce. When it is impractical to watch the far side drivers as the engine creeps over the bad spot, use a handmirror behind the engine tilted so that you can watch what is happening.
As for the rail ends, it is very important to dress them with a small metal file. You should get in the habit of imparting a gentle bevel to both the top surface and on the inner flange surface of each and every rail end...without fail. It isn't ever going to be a huge deal on the tangents, but you can see how a widish gauged axle going around a tight curve will want to force a flange to nudge and catch at slightly wide gaps, and if the metal rail edges are sharp, it will mean a displaced axle for sure.
I have never failed to locate and fix problems of this kind. It takes determination, and some cardstock or thin plastic shimming material shoved under several inches worth of ties and you will find that your engines will eventually behave, even at speed. If you have multiple areas of trouble, you may have to take up your track entirely, prepare a new subroadbed, and then re-lay it. It will take a weekend, perhaps, but if it leaves you enjoying the hobby more, especially a new engine, then it would be worth my trouble, in my opinion.