Brian covered many of them, here's my list:
Here in Southern California, we have some Pacific Electric substation buildings still standing.
Pasadena Sub, which (as I recall) started as a generating statiion, is now used by the city as a storage and workshop building. It's visible from today's Gold Line electric railway. (1951)
Altadena Sub is now an office building. (1941)
North Hollywood Sub is now medical offices. (1953)
Ivy Sub (in Culver City) is now a theater for a local acting troupe. (1953)
Vineland Sub (Baldwin Park) is a vacant brick building. (1951)
Los Nietos (southwest of Whittier) may be part of an industrial facility. (1951?)
Olive Sub (on Sunset Blvd in the Echo Park-Silver Lake are) not sure what it is now--was a taxidermy shop for many years. (1953)
Toluca Sub (at the portal of the Subway Terminal tunnel) now part of an apartment complex. (1955)
There are also some Los Angeles Railway (the narrow-gauge "Yellow Car" system) subs surviving:
Plaza Sub (across the street from LA Union Station, next to Olvera St.) now a workshop and storage space. (1963)
Soto St. (Boyle Heights) not sure what it is now--it was unusual in being halfway between two streetcar lines; Soto St. did not have a trolley line. (1963)
Huron St. (Lincoln Heights) not sure if it's still standing, was proposed to become a community center. (1956)
(year in parentheses is when the DC power was de-energized)
An interesting fact (if you're an electric-utility buff) is that all of these subs were fed by Southern California Edison power lines. Even those that were within the City of Los Angeles were not powered by LA Dept. of Water & Power, but by SCE. This dates back to the days when Henry Huntington established Pacific Light & Power to supply electricity to both PE and LARy, and was still true when the last streetcars ran in 1963.

It has been a while since I was last there....but the Washington Square substation of the Lehigh Valley Transit Co. was still in existence on Route 202 (DeKalb Pike) in Pennsylvania. This was part of the Liberty Bell Route of the LVT and approximately 38 miles from Allentown, PA
Keith
Google maps...if you go to street view you can see the brick substation at the intersection of DeKalb Pike and Township Line Rd in Norristown.
https://maps.google.com/maps?q=DeKalb+P ... CCoQ8gEwAA

I know of at least one Twin City Rapid Transit substation that exists but converted to another use.

The big one that still exists is TCRT's "main steam station" located at St. Anthony Falls in Minneapolis. The steam station provided power to all of TCRT's lines until June 1954 when the last streetcar ran. It was then sold to Northern States Power Company who continued to use it as an electric generating plant. It was later sold to the University of Minnesota who continues to use the plant to provide steam to the main UofM campus buildings. As far as I know it does not generate electric power.

Thanks!

The old power house for PTC for the route 6 trolley still stands, use Google street view:
1601 Church Rd, Wyncote, PA ‎
extensively remodeled but you can still see the trolley track path leading to Glenside.

The REI Sports building in Denver, CO is the former Denver Tramway power house in use up to 1955 as a trolley coach power source. It then housed the Forney Museum until the museum moved and REI took over.

May we expand this to include mainline traction? I would suppose there must be quite a few MILW subs are still around. Any GN, N&W, VGN?

Steve

The CA&E Substation from the Batavia Branch now exists at the Fox River Trolley Museum, although they currently use a rectifier to power the museum. There's another CA&E Substation building in South Elgin, now housing a Model Railroad Club. There's a third just off Mannheim Rd now used by ComEd.

The Iowa Traction uses some ex North Shore Equipment as their Sub, although I don't recall where is came from.

There are a few Illinois Terminal remnants visible from I-55 as you drive south between Chicago and St. Louis.

Those are all I can remember at the moment.

Around Dallas, there are a few substations still standing. Real fast, I can think of four:

Plano, TX, just north of Dallas, has a TX Electric depot and substation that is now a museum of the Texas Electric with a TE car on display and a full museum inside the depot.

Terrell, TX, just east of Dallas on US 80, has a small substation outside of town from the Terrell Interurban, a part of the Dallas Railway Streetcar system.

In Oak Cliff, a part of SW Dallas, has a Dallas Street Railway substation, less than a mile from my house on Tyler St. Is is used as an attorney's office and maybe the attorney/s house.

The old shops for TE had a substation within it if I remember right. The building is now used by DART.

I remember others in Dallas but can not recall the exac tlocations at the moment.

There are a LOT of substation buildings still extant that are out there. Additional CA&E substation buildings exist in Clintonville (home to the Valley Model Railroad club since the CA&E was still operating) and at Prince Crossing, northwest of Wheaton.

The below page also includes photos of numerous Illinois Terminal substations that still stand including ones in Bement, Bondville, Bloomington, Danvers, Fithian, Girard, Harristown, Mackinaw, Minooka, Morris, Union (but not THAT Union), and Utica.

http://hickscarworks.blogspot.com/2008/12/illinois-terminal-photo-album.html

Edit: Speaking of "that" Union, while the original (and significant, as I believe it was the first automatic substation built) E&B substation building in Union is long gone, the next one to the west in Garden Prairie still survives.

The building from the Northern Electric's Del Paso Substation was dismantled and moved to the Western Railway Museum where it serves as one of our substations. The rectifier equipment inside came from BART's test track in Concord, CA.

WRM also has Sacramento Northern portable substation #1, which is a rotary converter mounted on a flatcar with a boxcar-type body built around it.

http://people.virginia.edu/~ggg9y/substations.html

The former power house and car barn for the New Bedford & Onset Street Railway on the harbor in Wareham, MA, are now the upscale British Landing condominiums and yacht club.

http://marinas.com/view/marina/4624_British_Landing_Condominiums_and_Yacht_Club_Wareham_MA_United_States

The old street railway buildings are on the site where the British landed and attacked the village June 13, 1814 -- hence the name.

At Salunga, Pa., between Lancaster and Mount Joy, Pa., is the combination substation and station of the Conestoga Traction Co. Elizabethtown line.

The Lehigh Valley Transit substation along Route 202 between Norristown and Center Square still stands, though vacant at the moment. The Lansdale sub is also still intact, and I believe the one at Sellersville is as well.

I was photographing Illinois terminal substation at Bondville when a husband and wife became suspicious of what I was up to. I explained to them my interest and it was a revelation for both to find out during our conversation what the history and the purpose of the building originally was, as they were using it as a repair shop for their agricultural equipment. In explaining the scope of the former IT, they expressed a strong interest in learning more as well as preserving \repairing the building. I put them in touch with the IT Historical Society and it's great to find that today it's in good shape.
All of this brings to mind while a building may be extant, that does not mean anyone local knows anything about it. If I remember the location correctly, there is a former CNSM substation sitting in someone's backyard at the crossing of Five Mile Road in Wisconsin.
I wonder if they know what it is as well...you just never know.
During that same trip to the IT convention, I made the acquaintance of a fellow ( whose name I have forgotten) that owned a IT substation who was in search of materials for repair. As I recall it, he was a piano teacher whose goal was to create a museum and had talks with NS (?) over acquiring some of the abandoned tracks....I never heard of what happened to his dream of a museum. Unfortunately I cannot recall the location...Does this ring a bell with anyone?

The building for Sacramento Northern's Valdez Substation on the abandoned Holland Branch survives.

I just remembered another Los Angeles area sub that was neither PE nor LARy--the Glendale & Montrose sub in Montrose. Passenger service was abandoned in 1929, but electric freight service with motor 22 lasted until about 1940, being operated by Union Pacific. When I looked for the building about three years ago, it was still in use as a residence.