Very good advice here already. I would emphasize the polishing of the main axles and the tender axles. It's not hard to jack up the journals and sand down the rust which is 99% likely there. This is also an opportunity to spread grease on the axles so when you lower the journals back down there is already lubricant there (likely to be next to none left at this time). Since track has to be laid to move it, I would suggest leaving it after the move to give the display some completeness (and the ability to move the engine, etc. in the future for more work, if needed).
When we moved SP1215 from its "plinth" we had a large loader tractor to haul the tender, but it couldn't budge it until we used a "car mover" pole (long wooden pole with a small "shoe" that wedges up a wheel tread) and with that wheel/rail rust bond broken, it rolled smoothly.
Since we were unable to remove the side rods & main rods, we cleaned the crossheads, used a hand grease gun to lube the rod journals, and unscrewed the cocks, put a garden insecticide sprayer up the holes full of oil and sprayed the insides of the cylinders. Also, if you can't drop the valve links/mechanism, open the inspection caps and spray oil in there too.
Done carefully, you should be able to just push/pull the engine with all the wheels rolling. No need to "grease the rails" for sliders.

Out of my own curiosity, what if someone needed to slide an engine, in order to move it to a spot where they could then get under the locomotive to service the journals and drop the rods, in preparation for getting it ready to roll again.

What preparations should be undertaken for the purpose of sliding an engine?

Grease, LOTS of grease on the rails; if you can jack up an axle, grease under the wheels too before moving.
Unless you have a Pit nearby, you are pretty much stuck with working on the ties, not many reasons I can think of to slide an engine to a "new" spot to crawl under it. When we did SP1215, the track had settled into the ground enough that the rail ends were almost touching the pilot footboards, yet we could work under it. 1215 is an 0-6-0, so ALL the engine weight is concentrated on the drivers. No matter how you go about it, it's going to be uncomfortable, dirty work under there. When the locomotive on the trailer left the park and hit the paved road, dust and dirt flew off like a cloud. Wish the photos were handy, it was quite a scene. You get to work around and in all those years of accumulated dirt, plus whatever was there when the engine was placed (not all engines were cleaned up before placement).

When you've got two locomotives side by side; and the rails underneath are filled with dirt almost up to the height of the railhead... and "two" locomotives is, well, a lot more than two... My idea is to set up a dedicated work space so that forklifts can move around the locomotives and help with handling rods etc., and also clear the space between the rails and maybe lay some plywood, so that guys don't have to spend a month crawling on ballast. But these engines will have to be moved to this spot first.

Would it be worth it chaining the wheel bus to the frame, through the spokes during the slide?

In the past when I've worked on the journals of park engines, corrosion would form in very specific areas, namely right past where the crown bearing ends, at about the 10 and 2 position, so I'd hate for the wheels to turn and for that rust to now end up under (and damage) the crown brass.

OK, that's not the "usual" park display situation, I can see why you'd want to have more work space. Gee plywood instead of dirt and ties?? Next thing I know you'll want a pillow down there too! :) Oh, and I suppose nice work lights too, eh??
Yes, that rust streak, you really don't want it going through your brasses. The tender journals are pretty easy to get to, so I'd do them before the move. I have seen journals still clean after decades of display; lots of oil and a tight sealing lid works wonders, as does a lack of water being sprayed on the trucks (grass sprinklers, etc.)
Chaining the wheels, I don't know, your question has gone above my pay scale! But not letting the axles turn and damage the brasses is important.

Regarding the replacement crossties . . .
Spoke to an ex-MIGN section foreman who worked between Elmira and Mackinaw City on what had been PRR track. In the late 1970's he found untreated white cedar ties that were old enough to have been sawed only on two sides. Badly plate cut (actually rail cut, since there were no tie plates), he changed out the ties for that reason. The ties showed no signs of decay.

Suggest you check with local sawmills to see if anyone is bringing in logs from the cedar swamps in your area. For stationary support, if the cedar ties are available and the price is competitive with used crossties, you might give cedar consideration. I have had limited experience with them. You can show off by carrying one on each shoulder and they are easily spiked. They will last indefinitely.

Alex Huff

One sliding trick I read of was to put a leather glove in front of/under the wheels - and lots of oil or grease. This prevents wheel damage. Used for diesels with a locked traction motor.