While up at Cass this weekend I talked to one of the employees and this was his response.

"The point that was made in a few of the postings is that we have to climb steep grades and therefore need the boilers to provide maximum steam pressure to meet the demands. This is so true and steam is also needed to operate the air pump, injectors, lubricators, dynamo, whistles, etc. When one of our locomotives makes it's way up the mountain it is consuming a great deal of water. There are times when both injectors are operating. The grates need to be covered with coal to prevent cold air from striking the firebox sheets and flue sheet. If a fireman does not keep up with the demand, the locomotive will soon have to stop and build up steam once again.

There are many steam locomotives on tourist railways that do not operate on steep grades with sharp curves. Therefore, the crew does not have to maintain the fires and boiler pressure as we do at Cass. Black smoke is not intentionally made to put on a show. A fireman watches the black smoke leaving the stack, and when it starts to gray out, he will begin to throw coal once more. The goal is not to conserve coal, but rather to handle the demands of a mountain railroad. As pointed out in one posting, when the locomotive levels out in a switchback, the safety valve may lift. The engineer has closed the throttle, so steam is temporarily not needed for power. The steam pressure will rise slightly, setting off the safety valve. However, the locomotive will soon be back in motion and will still need a hot fire to handle the demand.

The coal that we use is top quality West Virginia coal that has high btu value and low sulphur content. "

Walter, you and I must have talked to the same person. I got, more or less, the same answer. Basically, they're firing every 90 seconds, and the smoke never really has a chance to clear up fully before another shovelful goes on.

The firebox reaches its hottest point at about mile 2 - which is about a mile into the 3-4% climb between Back Mountain Road and the first switchback. At this point, he said, the stack typically clears up to varying shades of gray and varying levels of density - and stays that way on and off through the second switchback. Since the train is passing through a less-photographed area that is mostly covered in tree canopy, we rarely see or notice it then, black smoke or not. The smoke will thicken up around Gum Curve, which is a bit steeper, as well as just below the second switchback. In the last few yards above the second switchback, the grade steepens up to 9%, and the smoke darkens accordingly with the rate of firing. For your average photo shot arriving in or departing from Whittaker, there will be thick smoke. Then, with another long 3-4% pull along the side of Back Allegheny Mountain between Whittaker and Old Spruce, the fire will clear up again (somewhat), but again in an inaccessible location mostly covered by tree canopy.

Most excellent, thank you all for your responses. Just a case of needing to put some black in the stack to get the work done :D