A friend, knowing I am a train nut, sent me a pic of ATSF 3759 on display beside their hotel.
What is the red thing on the stack?

Cheers from Dallas TX!
-Cp.

That's actually ATSF #3759, not SP. The red thing is a stack extension designed to direct smoke in such a way that it doesn't enter the cab. Why it's painted red now is anyone's guess. Kingman, AZ, has their city name stenciled on this locomotive in at least a half dozen places, maybe so if it gets lost the finder will know where to return it to (!)

OOPS!
Post edited, Thanks Dave, I was so interested in the red thing that I missed the enormous "Santa Fe" logo on the tender.

The red device is a smoke deflector. It is air operated and normally stored retracted in front of the stack. The purpose of the smoke deflector is not to vent smoke away from the cab, but to redirect the blast of the steam exhaust away from the lining of a tunnel. The biggest user of smoke deflectors and smoke ducts was the Cincinnati, New Orleans and Texas Pacific Railway due to the number of tunnels on their line. All of their locomotives were fitted with deflectors and many were refitted with Wimble Smoke Ducts. Due to its size, the Wimble smoke duct was mounted on the sand dome.

The only way to effectively vent smoke away from the cab in enclosed spaces like a tunnel is to extend the smoke stack back over the cab roof. Southern Railway actually did that on one steam locomotive. The Southern also experimented with supplying compressed air to the crew using gas masks. Neither experiment was considered successful.

I thought the red paint could be primer. The engine’s paint does look a bit less faded than in years past. That red paint however just looks too red to be a primer. The only modern engines I’m aware of that had red smoke stacks were NC&STL engines. I’ll link an older discussion on why the NC&STL stacks were red.

viewtopic.php?f=1&t=32466&start=45

No... it's not primer. Just another injustice done to a park engine by a well-meaning person with a paintbrush. Check out this other picture of #3759 shortly after the smoke deflector was painted.

http://www.railpictures.net/photo/442230/

And check out the "creative" use of white paint on the pilot. Ugh! At least Kingman cares enough to keep her painted.

I always thought one should need a license before one can use a paint brush.

You misspelled "can of spray paint."

I have been told, Grand Canyon Rwy. once approached the city about purchasing or leasing the Northern, and returning her to service. The city turned down the offer, as Kingman likes her just as she is. The locomotive, like all park engines, needs some help from rivet counters like us, but overall, she's in good condition and gets attention. There are few I've seen that are as well -off as Kingman's loco.

Tucson's ex-SP Mogul No. 1673 is in better condition. She was removed from Himmel Park to the train station in the 1980s and sits behind an attractive iron fence under a permanent canopy. The locomotive is owned by the City of Tucson, but they permit the Southern Arizona Transportation Museum (a division of Old Pueblo Trolley, Inc.) to maintain it per agreement. The Museum allows the public access to the locomotive on weekends or by appointment and has been striving to replace missing components and maintain the locomotive in good cosmetic condition. Interpretive signage in English and Spanish is provided.