Martin Underwood:
The first underground line was the one from Paddington to King's Cross -

Actually the first section opened continued to Farringdon.

what is now the Metropolitan/Circle.

No; it was originally the Metropolitan Railway, but in today's nomen-
clature it's part of the Hammersmith & City Line. Most of the original
route is also part of the Circle, but not at the Paddington end, where
the two lines split; and as for the Metropolitan Line, of course that's
now considered to branch off at Baker Street.

This was opened in 1863. I believe it
was originally driven by steam locos which condensed the steam rather than
releasing it into the tunnel.

The condensing didn't work so well once the Circle Line (then called the
Inner Circle) was opened in 1884, because there was no chance to stop the
trains and drain off the hot water. Nevertheless, steam working continued
until 1905.

I'm not sure what they did with the smoke...

They released it into the tunnel, and the management claimed that it was
good for you. (It smelled bad, so it must be, right?) One time at an
inquiry, a driver reported that it "very seldom" got thick enough to
obscure his view of the signals.
--
Mark Brader "Great things are not done by those
Toronto who sit down and count the cost
of every thought and act." --Daniel Gooch

My text in this article is in the public domain.