What is going on here (electrically)?

When you said Philadelphia I immediately thought Scott "T" but that just looks like it might be a boost transformer to pick up a sagging voltage or something. It is certainly nothing I see on a pole in Florida. I am curious what others say.

Hello, and it's hard to tell from the photo refs you provided. Sometimes the MV bushings on pole-mounted distribution transformers are mounted on the sides of the can. Also, a single MV terminal on the top of the can (typical for MV line-to-neutral connection) might not be visible in the photos. Like you, I get curious when wiring practice doesn't at first appear to follow the 3-phase, 4-wire, multigrounded neutral MV distribution system used over most of the U.S. Sincerely,

No, there were no MV connections on the small cans. I looked when I was there. I noticed because it looked so strange. Look at the last image, there is no MV present on the pole at all! That MV line ended one or two poles away at another (large) transformer.

Some research tells me that Philadelphia still has a fair amount of two phase customers, which are mostly fed by 3 phase to a Scott-T transformer setup. I don't know if that fact has anything to do with what I saw. The neighborhoods where I saw this were very old residential areas with some small businesses (restaurants etc.) and I don't see a need for 2 phase or 3 phase in old residential row houses.

That is why I suggested they may just be boost transformers to prop up sagging voltage. I have never seen anything like that so I am just guessing.

I have never seen anything similar elsewhere. Why a boost transformer directly below a main transformer (and connected to it) rather than correcting the transformer's voltage itself?

I have seen regulating transformers on MV lines, usually on lower voltage (older) lines running a long distance.

Distantly related:

Buffalo NY (one of the first cities to be electrified BTW) once had an OLD system of street lights with incandescent bulbs wired in series. I believe this system is all gone now but am not sure. Somewhat of interest was the setup used to power a particular circuit of lights. There was a large cylinder can (almost certainly a transformer of some sort), a small cylinder and a rectangular box. I believe the setup as a whole functioned as a constant current regulator. I also think there were devices in each lamp in parallel with each bulb that would short- circuit if a bulb burned out (the burned out bulb would be subject to the full string voltage momentarily and the device would "fail" and short out) and the rest of the string would remain on. Think of it as a reverse fuse.

Transformer cans - I wondered about power factor correction caps. But all of them I have seen are on the distribution voltage side.

Buffalo got power from the new AC generators at Niagra Falls - the success of AC over DC in the "war of the currents".

Early electric lighting was arc lights. I think they were series.

Airports can have runway light circuits that are miles long. I think (US) they are series loops at constant current. May be fed at 2kV or higher. Taps to lights are with a current transformer. I assume they must have a shunt for burned out lights.

Hello, and again looking at those pics they all have another thing in common - a street light. Most U.S. street lights operate at 120 VAC with some at 240 VAC. Usually that's supplied by a nearby MV-LV distribution transformer. Perhaps the lamps in those photos use some other voltage that's being supplied by an LV-LV transformer. Sincerely,