A loop-to-loop O27 layout can fit on a 30" x 79" door and has much more interesting action than a plain oval. Non-derailing switches provide an automatic "random-route" feature, so the train runs through the central junction in different directions.
I currently have a temporary layout set up with this plan. I use standard-O switches (#O22 switches with O31 curves) which have a superior non-derailing function. Non-derailing O27 switches might overheat with the train going over them so frequently.
Standard-O track is easily connected to O27 track simply by forcing standard track pins into the O27 rails. It may help to spread the rail web slightly with a screwdriver before inserting the larger pins. The O27 track needs some shimming to get up to the height of the switches. The O27 curves allow this plan to fit on a standard-size 30" x 79" door.
conventional tubular track
The track overhangs the table ends slightly; you could tack 1x2 strips on the ends to extend the table. Some track is cut to fit. The 5.8" straight sections relieve the S-curves and require only a little extra length in the plan. Add spurs and more track if you like.
