About 25+ years ago my wife and I rode a 611 fantrip up Saluda. It was said that the grade is so steep that the engine had to run up the grade to assure that its firebox was always surrounded by water, thus avoiding any risk of a boiler explosion. Not sure of the truth in that, but it makes some sense. The train went on to Asheville and then via the Old Fort line on its return.
We also rode two 1218 trips in that era, in NC and AL. As passengers we could actually feel the gentle tugging of the two "engines" under the 1218 as they'd run in and out of synchronization depending on load. It was pretty cool.
With the current legal climate, associated insurance costs and escalated ticket prices, we're glad we rode when we did.