Start by printing track. I left holes to screw the track to plywood.
The turning pieces (30 degrees each--3 of them make a 90 degree turn) are cut off from the straight piece (using Autodesk 123D--free).
The diameter of a complete circle is 12.5 inches. Make the top longer, but use the 15 degree cuts at each end, and you will achieve a larger radius. Change the angle if you want something completely different.
You can clip the pieces together using a printed clip (I only designed the corner clip--corner to straight and straight to straight will be longer clips).
Because the seesaw must be held firmly in place (and the approach pieces also), I used 1/2 inch #4 woodscrews.
Two straight pieces of track are held together using the pivot piece. The pivot piece is not centered--so that the track will tend to rest in one direction.
Press the 3/8 inch (ID) bearings in the support pieces (vxb.com for bearings).
Cut a 3/8 inch diameter wood rod (72mm long) and press it into one bearing, through the pivot piece and into the other bearing.
To make the seesaw reliably land on one end, I glued a chunk of surplus plastic (a bad print) under the right side.
Set up your layout and let the train go!