You can always tell an oil-burning steam engine from a coal-burner without much difficulty. Both oil and coal-fired engines pretty much sound the same underway, and both are fully capable of blackening the sky. But the oil-burner will be the one with the bright, rapidly flickering flashes underneath the cab, looking like she's about to set fire to the track. This is because the design of the combustion chamber on an oil-burner is so totally different from that of a coal-fired engine. A coal fire consists of a relatively even bed of burning solids that sits on top of a set of grates. On an oil burner, there's a continuous flow of liquid fuel, which is atomized into the combustion chamber by jets of steam. The size and geometry of the fire can vary considerably from a small spot-fire when the locomotive is stationary, to a wide, raging torch underway. The design of the burner apparatus is also such that the fire is much more visible from the outside, often producing bright reflections off the ties and rails below. Sitting still, there's also a more perceptible roar from the firebox than with a coal-fired engine. Firing with oil is trickier than with coal. The Engineer and Fireman have to coordinate throttle movement and firing changes closely, or bad things can happen, including flame-outs, and even explosions. A poor job firing can definitely cause maintenance issues down the road. On the good side, oil-burners don't shower the photo-lines with cinders, and if you're lucky enough to get cab ride in one of these babies, you won't end your day looking like you've been rolling around in a pile of soot.