This is my very first project in OpenSCAD. Tracks are designed from scratch, the goal was to shorten print time and to increase bottom surface - to avoid delamination of ABS from the hotbed.
One try was attempted to print the straight track, but was unsuccessful due to printer failure - new attempts will follow next week.

Note: you need OpenSCAD 2016 to modify/render the curved track, 2015 has no angle parameter in rotate_extrude.

2016-03-09: All files were updated (actually, the old curved track was not printable) - I have printed several copies of both rails. They connected smootly to each other and to the original Duplo.

2016-03-18: Increased width: distance between rails from 27 to 28 mm, rail thickness from 1.5 to 2 mm because my dauther have broken one 1.5 mm rail, track width from 7 to 9 mm - because one wheel could drop down from the old version of the straight track. Sorry guys, I have not so much time for testing, and my children do not report bugs immediately :)