Track can be curved as tight as your equipment can handle. Four axle (GP-9, S-2, etc) and shorter wheelbase engines can handle a tighter turn than six axle units. For instance a Life-Like SW-1200 can run with cars through a 5" radius before the car ends meet. (but you can't couple on a curve like this) At around 8" just about everything should fit but be sure to avoid the flextrack bending out of a flat plane. Also watch the engine trucks to see if you are getting into a bind, this can cause it to eventually jump when you aren't looking.