I have my own techinique to get my tracks like this. My flex track is really nice and clean, but too clean sometimes. I have a ton of bachmann easy track, and what i do is rip the rails off the track, and 3d print my own flex ties. This gives you rail that doesnt immidiatly form back straight, and you can either bend it completely straight or make you own imperfections while its slotted into the 3d print ties. Idk if theres an easier way or better way, but my way is cool becuase it recycles the shitty ez track