When using the LEGO rollercoaster track system (Creator 10261, 31084 and others) and adding a looping using 2x four curve-elements you will notice that one need a straight with 4 studs length to compensate, but LEGO offers only as 8 and 16 studs length.
So I took a caliper, measured all lengths of original track and hacked this into OpenSCAD (starting with 2x2 flat block as first test), and for some reason already the first printed attempt worked with original track system.
You will need to print with support on bed, and then possibly remove some material from bottom inside with pryer to get better fit on studs.
Happy printing and building.
Print with support only on heatbed, see third picture.
I used Cura 15.04.6 to slice this using profile derived from original Anycubic profile (imported from owl model coming with SD card).