David Lesher writes:

The other "cans" you often see on poles are capacitors used to adjust
the power factor on some secondaries.

Capacitors are in various places but we also have three 7200V line
regulators a block away, one on each primary phase. They are
auto-transformers, with allegedly auto-controlled tap changers, much as
the other poster described. [But his description is more complex than
I recall from the class covering same. The essential aspect was you
CAN short two taps together while switching; the inductance limits the
current change while you do..]

I say "allegedly" as twice now, the regulators have stuck and my UPS
woke me up at 2:30AM with notices it was disconnecting from the now-128v+
line. I solved the issue that night by putting a Variac in the line
ahead of it, and cranking it down.

I also asked about tap changers/regulators. Is this what these are, and
if so, are they likely to be manual or automatic?

http://tinyurl.com/534ffq (Google Maps street view of a set of 3 on a pole
in upstate NY. You may have to click on street view and rotate to see them)