I am looking at the new GP40 premier diesels and notice they have 2 operating smoke units. Can someone tell me what the 2 smoke units are for? Assume one is for exhaust of couse. If it has 2 exhaust ports I would think one smoke unit would do fine in a 15 1/2inch switcher. Just wondering what the 2nd smoke unit is for. I like smoke and have been wanting a premier diesel that runs on o31 track. I don't know much about the GP40 diesels so sorry for my ignorance.