My little granddaughter (http://www.thingiverse.com/thing:61422) is almost 2 years old now and she has got a new Lego train. The new system does not need powered rails any more, so why not extend the tracks a bit in nice colors ?
Be aware, still w.i.p. to get measures right, I am waiting for the first print so don't download/print it yet!
This version does not fit, as I have switched the connectors :-(
I slice with 0.25mm layers and one solid layer every 15.
The lego compatibility is not on the bottom side. The track has a solid bottom.