I didn't mention this, but if you are using 2x6 studs, use 2x6 headers and bottom plates as well. I think for strength, you should double your headers-ie put 2 2x6 boards on top of each other, screwed and glued together for strength. On the 20 foot walls, stagger the joints. Depending on whether or not you are going to use the roof of your railroad room for storage would depend on what you need to do for ceiling joices. If you are not going to try to store anything above the room, you can use 24 inch spacing for your joices. If you want to use the space for storage, use 16 inch spacing for the joices and put in a roof of 1/2 inch plywood or thicker to support the load.