Butter a 13 by 9 by 2-inch cake pan. Line the bottom with wax or parchment paper, butter the paper, and dust the pans with flour, shaking out the excess.
Preheat oven to 350 degrees F.
Into a small bowl, sift together the flour, baking soda, baking powder, and salt. In an electric mixer fitted with the paddle attachment or a hand held mixer beat together the sugar, butter, and the peanut butter until the mixture is light, creamy and combined well, about 2 to 3 minutes. Add the eggs, one at a time, beating well after each addition. Beat in the sour cream, vanilla, and the water. Add the flour mixture and mix until just combined. Transfer batter into the prepared pan, smoothing the top. Rap the pan three times on a hard surface to expel any air bubbles. Place the cake in the middle rack of oven, bake for 20 minutes, rotate the pan, and continue to bake another 20 minutes or until a tester comes out clean.
Let the cake cool in the pan on a rack for 10 to 15 minutes. Run a thin knife around the edges of the pans, invert the cake onto the rack, discard the paper, and let it cool completely. The cake may be made one day in advance and wrapped well.
Make the frosting: In a heavy bottom pan slowly heat the heavy cream. When the cream just comes to a boil pour it onto the chopped chocolate. Add the corn syrup and mix well with a wooden spoon or rubber spatula. Let mixture cool for about an hour or until it is firm enough to spread.
Assemble the "train": Trim the edges off of the cooled cake to make a perfect rectangle about 12 inches long. Slice the cake in half horizontally into 2 layers. Spread the jam on the bottom layer and place the remaining layer back on top of the jam. Cut the cake directly down the middle into halves and cut again into quarters. At this point, there should be 4 equal pieces of cake. Continue to take each quarter of the cake and cut into 3 equal rectangles. You should end up with 12 -2 inch wide pieces of cake. Take 1 piece of the cake and set aside. Frost each piece of the remaining pieces with the frosting. Take the reserved piece and cut it in half. Arrange it on top of one of the other pieces to form an engine car. Once the cake is frosted, arrange them like a train on a licorice "track" and decorate them with the candy and nuts. Serve the cake with ice cream.
Recipe courtesy of Gourmet Magazine