Regardless of preference of appearance, the point of the various styles of stacks other than those that were straight were to reduce trackside fires from cinders and sparks escaping. while I like the appearance of the Radley Hunter "diamond"spark arresting stacks, the Rushton style was what SVRy decided was optimal after some years of trial. Most railroads across the country had moved away from burning wood as fuel long before SVRy, so that the fact that they used the "diamond stack" into the mid 1920's is probably related to their best attempt to reduce track side fire. Coal burners added netting to smoke boxes to offset this danger while trying not to interfere with drafting. Now if you took a 19teens era engine and installed a Congdon stack, that would seem out of place. How late did railroads use the Congdon style stack? Like SVRy and the Radley Hunter Stacks in the 20's there must have been some late hold outs on the condon style?