Here are a couple of crusting buttercream recipes that you could use for your Thomas cake.
This frosting will need to be beat on high for 8-10 minutes, so you'll need a high quality upright mixer. I use a Kitchenaid to make my frostings.
2 lbs powdered sugar (I use most of a 2 lb bag of C&H powdered sugar... leave about 1/4 cup in bag) Never use beet sugar!
1 stick butter (this measurement is the same as... 1/2 Cup, 4 oz, or 113.4 grams), room temp (do not use margarine or butter substitute)
1/2 cup of Crisco (regular, not butter flavored) or hi-ratio shortening
1/2 cup of milk or water
1 tsp clear vanilla extract (you may use regular vanilla extract, but if making white icing will give it a bit of a tint)
1/4-1/2 tsp almond extract (imitation or regular)
1/8 tsp salt
Beat together butter and shortening in your mixer, add vanilla, almond extract, 1/4 milk or water (half of that called for in recipe), and salt. When well mixed, add sugar a cup at a time. As frosting gets thicker, add enough of the remaining milk or water until the consistency seems right. You may or may not need the entire 1/2 cup. If it's humid or raining, you may need to add all the liquid, possibly even a tsp or two extra.
Scrape sides of bowl and mix on high speed for 8-10 minutes. It's a good idea to cover your bowl with a warm damp towel to keep the frosting from drying out.
This frosting may be stored in the fridge for 2 weeks, or in the freezer for 6 months.
Another frosting recipe that works well for decorating a cake is Wilton's class recipe:
1 tsp Wilton clear vanilla extract
2 Tbs milk or water
1 cup Crisco (regular, not butter flavored)
1 Tbs Wilton meringue powder
1 lb sifted confectioner's sugar (approx. 4 cups) Never use beat sugar!
optional- 1/2 tsp. butter extract may be substituted for 1/2 of the vanilla, a pinch of salt may be added.
Sift confectioners' sugar and meringue powder into a large mixing bowl, set aside. Cream shortening, flavorings and milk or water. Gradually add sifted dry ingredients and mix on medium speed until all ingredients have been thouroughly mixed together. Blend an additional minute or so, until creamy.
If your frosting seems too sweet, add a pinch of salt. If it seems to thin, add a small amount of confectioners' sugar until it thickens up. If too thick, add a tsp of water or milk.