Measure the dimensions of the outside of the train table top. You will want your tabletop to rest on top of the walls of the train table, not directly on the play surface.
Cut 3/4-inch plywood to fit the dimensions of the train table. If you are shopping at a big-box hardware store, an employee can help you cut the plywood to your measurements. Sand the surface and edges of the plywood. Wipe away the dust with a tack cloth.
Decorate the surface of your new tabletop. Try staining it to match your other wood furniture and then varnishing it, or paint it with a bright color or pattern. Another easy way to decorate the tabletop is to decoupage it. Cut up small pieces of paper (thin gift wrap, scrapbooking paper, and sheet music all work well). Using a decoupage glue (such as Mod Podge), cover the table's surface with a pleasing arrangement of paper scraps. After it dries, seal it with another layer (or several) of decoupage glue.
Measure the perimeter of the table and the height from the tabletop to the floor. Most train tables are between 18 and 24 inches high, and are approximately 14 feet around the perimeter.
Choose and buy your fabric. Three yards of 54-inch wide fabric should suffice, if your train table fits the usual dimensions. Cut it in half lengthwise and sew the two halves together to make one very long strip. Hem the top and bottom so the fabric reaches from the top of the table to the floor.
Glue hook-and-loop fastening tape around the edge of the tabletop. Sew the matching part of the hook-and-loop tape around the top inside edge of the table skirt. This way you can hook the skirt on when you are using the decorative top, and take it off easily for washing and storage.
Use a removable pressure-sensitive adhesive (such as DAP BlueStik) to hold the tabletop in place when you are entertaining.