Step 2: Making the Round Part:
The diameter of the round part needs to be as wide as the top bed you made in step 1. Make sure you cut the circles out of the middle of the MDF (you will need the "scrap" piece to build the bumper of the train). I screwed together two pieces of MDF and then cut one cirlce to get exactly two cirlces the same size. You will then need to router out the inside edge of the circles to countersink the mason board you will use to wrap the round part. See See picture for how I put it together.