I use mostly Atlas flex track, and cut to fit. I solder almost all joints, with a few loose for temperature swings. I use Kester 86 no-clean water soluble flux and 60-40 Pb-Sn solder (fine stuff). I use a Yihua temperature controlled fine-tip soldering system, set to about 275C or ~500F. This will join almost everything on my layout. IF you use the flux, you should be able to make a clean joint in under 2 seconds, so the roadbed will not be affected. If you wish, you can rinse with IPA or Methanol, or water. Just blow the liquid off when done.