I was going with "FLUX CAPACITOR", but I think Mr. Anderson has it correct.

A drifting valve setup to release vacuum conditions inside the cylinders/valves.

Not totally necessary to make a steam loco operate, but a refinement that reduces stress and overall wear and tear on the parts. And it keeps outside "grit' from being sucked into the "moving parts"..

When the loco is drifting with no steam supplied to the cylinders they become a sort of vacuum cleaner and will tend to "suck up" everything around them into the cylinders....

When everything around the loco is dust/dirt/grime/cinders it helps to avoid "Hoovering" them into the inside of the cylinder.

Drifting valves. There is a piston under each three stud head, with an attached valve poppet. When pressure in the steam chest rises, the pistons force the poppets against their seats at the ends of the rolled in tube, and the engine operates normally. When there is a lack of pressure, springs open the poppets, opening communication between both ends of the cylinder, keeping a vacuum from forming that might pull cinders into the moving parts.

"Does this loco have a compensating valve to allow live steam to the LP cylinders when starting a train, or is it always compounded?"

Does this loco have a compensating valve to allow live steam to the LP cylinders when starting a train, or is it always compounded? Thanks.

I was going with "FLUX CAPACITOR", but I think Mr. Anderson has it correct.

A drifting valve setup to release vacuum conditions inside the cylinders/valves.

Not totally necessary to make a steam loco operate, but a refinement that reduces stress and overall wear and tear on the parts. And it keeps outside "grit' from being sucked into the "moving parts"..

When the loco is drifting with no steam supplied to the cylinders they become a sort of vacuum cleaner and will tend to "suck up" everything around them into the cylinders....

When everything around the loco is dust/dirt/grime/cinders it helps to avoid "Hoovering" them into the inside of the cylinder.

"The Wagner first appeared on the 3751 class of engines and was an integral part of the casting along with the cylinder and steam valve assembly. The Wagner is the very top “tube” like section above the conventional valve chest of the cylinder casting. This entire casting has special porting so the Wagner Bypass valve can close off the supply steam passage to the valve chest and open a passage between the exhaust ports on both ends of the steam cylinder effectively connecting the space on both sides of the piston together.

When the throttle is closed, a spring loaded piston valve in the Wagner moves over to connect the exhaust ports on both ends of the steam cylinder so the air [note that it's actually vapor] on both sides of the piston simply oscillates back and forth through the Wagner eliminating the need to supply steam to the steam cylinder to help cushion the piston while drifting. In addition, the Wagner closes off the steam supply ports leading to the valve chest. Basically, the steam cylinder is cutoff from the outside world. There was supposed to be enough of a restriction to the air flowing back and forth to each end of the cylinder through the ports that a small back pressure would be created and would sufficiently cushion the piston and brasses regardless of the speed. Slow speed, small backpressure. Faster speed, bigger backpressure or more cushioning.

When power is again applied by opening the throttle the steam travels to the Wagner Bypass valve where some of the steam is bled off to the exhaust nozzle but it begins to build up pressure against the piston in the Wagner, and after a predetermined pressure is reached it forces the piston against the spring and this reroutes the ports so the supply steam and the exhaust will follow conventional paths like a normal steam engine.

The theory was that there would be a substantial fuel savings because the fireman could set up a very light “spot fire” on long down grades that took a lot of time rather than having to fire for a drifting throttle working steam that was set according to the engineers technique. If you’re ever around an engine with a Wagner when they are starting to pull, you’ll hear a lot of steam making its way out of the stack as the Hoghead builds throttle slowly to get the Wagners to seat, and then suddenly there will be little if any exhaust and you’ll hear and see the steam blowing out of the cylinder cocks as he waits for the engine to react to the throttle."

"The Drifting Throttle is the standard where the Hoghead sets the throttle to feed the cylinders during slow and high speed drifts with enough steam to cushion the piston and brasses. Some of them used the backpressure gauge on the engine if it was equipped and most of the time they would just open the cylinder cocks and listen for the right amount of steam emitted for the speed they were drifting. On engines without mechanical lubricators it was a must to keep steam fed to the valve chest and cylinders in order to keep them lubed. This was standard practice on engines using hydrostatic lubricators.

I found a directive in one of the old Santa Fe brown books. According to the book, when you were drifting on long slow downgrades, they wanted the Johnson hooked up to minimize wear on the motion. However, the engineer was to drop the reverser down for a short distance once every ten miles or so in order to spread the lube over the entire face of the valve cylinder and force any oil that had accumulated there down into the steam cylinder. Those old boys had their thinking caps on.

The Drifting Valve was another smaller valve separate from the throttle valve and often fed from the steam turret in the cab with its output plumbed into the valve chest at a convenient location. I’ve seen some illustrations where it was preconfigured by the engineering department where it had small ports and the engineer simply turned it on or off as needed and lived with the flow provided by the fixed setting, supposedly. I’ve also read stories where the design engineers got it wrong according to the experiences of the engine crews and the Hogheads used a drifting throttle along with, or in place of, the drifting valve. They were not going to have their reputations clouded by handing off an engine that had the brasses burnt or pounded out of the rods to a relieving engineer.

I have combed the internet and a number of publications looking for information on these Wagner’s and any discussion on their proper use and details about their effectiveness that may be available . Talk about the Lost Ark of the Covenant. Scarce as a hen’s tooth.

Sad to say, the old fellow that I had as a mentor and actually worked the 3751 between L. A. and Needles, was from the old school when the 3751 class came along and simply ran the engine the same as he had run all the rest, drifting throttle. Wagner Bypass valves? “Didn’t use ‘em”.

I talked to an old roundhouse guy one time out of Albuquerque and it was his contention that they were to be used at any drifting speeds no matter what. I put that in the note book for future reference.

The only other info I could dig up was by inference from the book, Santa Fe’s Big Three, where there were exhaustive tests made on the 3400’s, 3751 class, and the 2900’s. They had a mind boggling amount of technical data, track charts, graphs, and tables that would suck your eyeballs out of your head after about five minutes but I battled my way through it a bite at a time and from what I can infer, they used the Wagner anytime they were drifting because it was a company mandated design intended to squeeze as much fuel economy out of the engines as possible.

When the Hogheads were on their own it’s anybody’s guess as to how many of them used it and how often, but when the dyno car was attached they were under the gun it was standard practice to use the Wagner.

The smaller external plumbing that you see is probably the drain piping from the Wagner, valve chest, and the steam cylinder. My understanding is the Santa Fe drained all these cavities individually to the cylinder cocks rather than allowing the condensation to work its way down to the steam cylinder and be expelled from there.”