When you're ready to permanently place Thomas' face into the cake, take a spoon of piping gel and smear it on the backside of his face. This will act as a glue and keep his face in place.
The wheel dilemma now presents itself. You'll see that the wheels on your cake are a bit small, if you make them the way I have. I haven't found a good solution for this. In the toy, the wheels are suspended below the base of Thomas' body. So, I've just made them smaller to fit the cake. You could decorate the cake in such a way that the wheels were larger if you wanted to make the other features thinner. I opted to do it with smaller wheels and a larger body.
When you're happy with Thomas, take some frosting and pipe out a track to make the look complete.
I usually keep shaped cakes cool in the refrigerator before transporting to a party. A cold cake will be more rigid and less likely to fall apart on you.
A secret to transporting successfully is to use the waffle like shelf paper (available at Walmart) between your cake board and the floor of your vehicle. This technique has never failed to hold my cake in place.