There are a lot of themes your toy train table can have. One popular theme is a military theme. Military themed activity tables can focus on a specific branch of the armed forces, or it can focus on an era. For ideas on military train themes and graphics I recommend reading the blog posted on Model Train Central. They have a great blog on military model toy trains.
The first option that you have for designing a military train table is to focus on the paint job. You can paint a military camouflage design on your toy train table. Camouflage comes in a variety of color combinations. The colors that are used are designed to reflect the colors of a specific environment. For example, a desert camouflage is going to be based on shades of browns and tans, whereas jungle camouflage is going to be based on shades of greens and blacks.
The second option that you have for designing a military train table is to use military stickers, graphics and images to decorate the table. You can find these items in stores, especially hobby stores, as well as in military supply stores. To affix these graphics to your table you will want to rely on something stronger than just the adhesive found on the sticker. Decoupage glue or gel, one of my favorite materials, works to both adhere the graphic to the table and protect it from moisture and damage.
The first option that you have for designing a military train table is to focus on the paint job. You can paint a military camouflage design on your toy train table. Camouflage comes in a variety of color combinations.