I always use packet mix – Betty Crockers Chocolate Fudge, because you can freeze it a couple of days prior, then carve and slice out the shape. And the kids LOVE the cake – seldom any leftovers!
I froze the cake, and cut out the trolley shape. I used off cuts to ice as the blocks. The trick was to ice in a big lump, freeze then cut into square shapes. I think if I were to repeat it, I would use a solid cake, like a fudge brownie for the blocks, as the spongy cake was difficult.
I used folded cardboard and toothpicks for the handle after several other ideas failed to adhere to the side because they were too heavy.