Nice job, but ^^what KJ356 said. I'm not sure if there's more than one version of the LionChief board for diesels. That's something you might be able to figure out by comparing parts lists / exploded diagrams on the LionelSupport website.
In the meantime I can tell you that "Thomas" kid stuff is all plastic and very light-duty; that board may not have enough current capacity for a dual motored diesel with lead weights (!) in the body. My prescription would be to ditch the weights and rewire the two can motors in series instead of parallel, if you didn't already connect them in series when you installed the board. This will reduce current draw, and you'll get better slow-speed operation as a bonus!
You can use wood or plastic to make a new fuel tank, or even just the sides; if they're thick enough no one will know that it's not solid all the way through. Good place for a speaker, and you'll add back a little of the weight with a lower center of gravity.
