In later years, 734 also had overfire jets. Three on each side. We used them for clearing up the stack when the engine was standing or drifting (never when working significant throttle), at times when excessive smoke was a concern. They were quite loud, but very effective. Before they were installed we sometimes used the stoker jets for the same purpose (turned up way higher than what was necessary to actually use the stoker), which worked but not as well.

Do they cause premature wear of the side sheets?

No problem at all. It's a scan from my copy of Steam Locomotive Diagrams of the Chesapeake & Ohio Railroad, compiled by the late Alvin F. Staufer in a small paperback back in 1964.

This video features 1309 stopping just before Helmstetter's Curve for a crewman to flag a crossing, then catches the 2-6-6-2 starting up the grade

https://www.youtube.com/watch?v=CiDFOEm622s

And here's another video, this one showing 1309 moving along at a good clip

https://www.youtube.com/watch?v=jNqlfYnuU6k

The exhaust sound is much lower than a traditional non compound, interesting to hear that.

Something like the 4014 or 1218 would have that characteristic double exhaust starting the train up again on the grade.

I’ve watched a few clips of the locomotive but I haven’t yet caught one where you can hear it switch from simple expansion when starting the train to compound expansion for running.

Has anyone caught that on a video?

That's because when the loco starts normally, the rear cylinders exhaust to the receiver, not the atmosphere. The exhaust passage from the HP cylinders to the stack is closed off. As a result, the loco sounds "normal" because only the LP cylinders exhaust to the stack, giving four exhaust per driver revolution. Only when the engineer opens the emergency valve (or sometimes called the simpling valve), then the exhaust from the HP cylinders goes to the stack, along with the exhaust from the LP cylinders, and you get the unique sound of eight exhaust beats per driver revolution. Interesting setup on the "old" compounds, which excludes the modified N&W Y's. And this is a highly simplified explanation of how it all works.

For a more complete explanation see http://www.catskillarchive.com/rrextra/mallet.Html

Makes you wonder if steam development had continued would more articulated locomotives have been simple or compound. Wasn't the planned NW y7 a simple engine?

N&w seem to have been one of the masters of compounding so interesting that on their next planned Y series they wanted to go simple.

I have to concede that I've only "lived" this via audio recordings (N&W Y6's by Link, and overseas compound Mallets), but:

What I have heard is compounds using "simple," meaning direct steam to all four cylinders, to get the heavy train moving, with eight beats per driver rotation unless they somehow magically "sync," and then as the engineer is confident he has the train on the roll, a dramatic cut from eight to four exhausts per rotation as the loco is switched from simple to compound for more efficient use of steam. You don't use compounding to get a train started--you need as much power as you can get from all four cylinders.

It's as if you were listening to a doubleheader, and all of a sudden the one loco disappeared.

Someone knows which Link recording(s) these were; I'm thinking it was the last track on Side B of "2nd Pigeon and the Mockingbird".......

I read that they’re still adjusting the simpling valve, so that might have something to do with the sound being off. That or there was just no video taken at a time that would’ve presented the engine simpling. Maybe whenever she gets out onto the line again and there’s more cameras we’ll hear it.

Either way, here’s where I saw that the simpling valve seeds fine tuning:
https://railfan.com/she-looked-glorious ... -maryland/

Ah, you're in luck, there are several instances of N&W Y's shifting from simple to compound (though only "2nd Pigeon and the Mockingbird" has the engine shifting from compound to simple), and that at considerable distance).

The whole of any of these recordings--the only complete examples I can find on the internet--are worth listening to, but the specific shifting sequences that may be of interest here are at:

"Sounds of Steam Railroading"--Sequence opening at 7:10 (switching with a Y at Waynesboro, Va.), departure in simple starting at 14:16, transition to compound at 15:45:

https://www.youtube.com/watch?v=UhBFD5JrKUE

No compound Mallets at all on "The Fading Giant," but a wonderful ride behind Js (one of them being 611, reportedly the only sound recording of her in revenue service), a 4-8-2 being moved for servicing at Bristol with a clanky, rattley turntable as a costar, M-class 4-8-0 382 with a beautiful whistle on the Abington branch, and the famous Christmas sequence at Rural Retreat, Va.:

https://www.youtube.com/watch?v=Sl4S4YKi70Y

Hooters call out, sounding like ghosts, accompanied by "Thunder on Blue Ridge." This must be an earlier edition of this record (it has some differences compared to the later one I was able to mail order from Link when he was still alive). It's hard to make out, but the first transition from simple to compound is in a sequence that starts at 4:45 (Link was standing somewhere near the center of the train as it got under way at Boaz, Va., the base of the grade); actual transition, barely audible (at least to me) is at 5:55 (it was interpreted as a slip by the fellow who wrote the liner notes).

At 9:00, we're about to go up the hill in the caboose as the pusher couples on. You can hear the intercepting or simpling valve flipping over a couple of times, eventually the train gets under way in simple . . .at 13:32 is a sequence where it sounds like the pusher drops into simple at 13:38. . .between 32:57 and 33:08 the engine goes back into compound (at least that's how I'm interpreting it). . .at 33:27 the rear brakeman pulls the pin and the pusher drops behind. . .there's still a bit more of the adventure, but it's not involving changes in steam flow, and I won't spoil it for you anyway!

https://www.youtube.com/watch?v=-LEjixDk5lg

There may be other sequences featuring this action on other records, among them those sold by the Railroad Record Club, but I haven't heard them and can't comment on them. Some of those are also on YouTube.