611, 765, and NS treated us to steam only motive power this year. 4501 tested out the mainline unescorted, and I think 630 ran from Knoxville to Chattanooga unassisted until a bearing got hot (2012ish maybe). These are some recent un chaperoned mainline events. How far back do you have to go to find it common to go "diesel-less"?

Well 4501 ran diesel-less in 91' and in years before that, I think 85-86 and 89. 630 ran some trips in 2012 without diesel including a ferry move from Asheville, and for the record she burned out a bearing in Nov. 2011. You would really have to look back to the 1980's for smaller steam to run without diesel. 765 and 614 on CSX and NJT ran without diesel normally. and 611 and 765 both ran without diesels this year, 611 has yet to have a diesel tied to her drawbar in her new excursion life.

While we're on the subject, what factors determine if a mainline steam excursion includes diesel assistance or not? Insurance premiums? Stiff grades? Dynamic braking? All of the above?

To add diesel or not.

1. Tonnage. 630 is not going to pull a 20 car train at 40 mph.

2. Range: How far can you go on a bunker of coal and tank of water with that train over that type of terrain? Do you have to travel 75 miles or 150 miles between water stops? Are there heavy grades where a diesel will greatly save coal, water or assist with braking down grade?

3. Does it really matter? Can we save $1000 on coal for a deadhead move and reduce water stops?

3. Contingency. The what if it breaks down on an especially busy main line?

Regarding Oil burners. I know UP toted fuel with them. Can the aux fuel tender be tied into a single supply like is done with a aux water tender?

I don't think any 'heavy oil' engine is equipped with multiple oil tenders. You would need modulated steam to heaters even if you were only doing 'transfer pumping' of the reserve oil into the tender bunker -- and I don't like the idea of unsupervised heating of oil even if it's not continuously 'on'.

Less of an issue if you're firing with light or waste oil, but I would still wonder about connections at low level between fuel tenders ... and of course you're complicating the piping and/or antifreezing heating for the larger capacity of aux water tender(s) needed to take advantage 0f the greater oil supply...

I don't think the UP carried their fuel with them. Anytime I have seen the tied up for the night, ther was a fuel truck topping her off.