Cover a cake board or platter with aluminum foil.
Place a small amount of buttercream in the center of the cake board and place 1 layer of cake on top. Cover layer with buttercream and add another layer of cake. Repeat for all cake layers and cover cake with a thin crumb coat. Refrigerate for 15 minutes.
While the crumb coat sets, color portions of the fondant dark blue, red and gray. Roll the fondant out and use a sharp knife to cut out train, wheels, and a "1" shape. You will likely need 3 train shapes, 6 smaller wheels, 3 larger wheels, and 4 "1" shapes.
Tint buttercream with blue food coloring gel to a medium blue. Tint a small amount of buttercream to dark blue for the lettering.
Frost the outside of the entire cake with the medium blue frosting. Apply the train, wheels, and 1's around the cake and place the last "1" on top of the cake.
Fill a pastry bag with a small round tip and pipe the lettering on top with the dark blue buttercream.
Fill a pastry bag with a small star tip and pipe a border around the top and bottom edges of the cake.
To finish, pipe a train track around the cake on the cake plate with the black gel icing.