Someone asked:
The [Central Line tracks] swap vertically between Chancery Lane
and St Pauls, IIRC. What is the reason for this?

I (Mark Brader) answered:
This one *is* due to a narrow street above.

John Rowland said:
I don't understand, since the do-si-do clearly occupies a larger
footprint than would two tunnels running above each other.

Ah, I missed the implication of "swap". I have three sources that all
confirm that the tracks run one above the other through those two
stations (as well as at Notting Hill Gate) because of narrow streets
that existed above them at the time of construction, but none of them
mentions *which* track is above the other, or why, and I hadn't realized
that it was different in the two stations.

As to John's point, the width limitation only applied to the stations.
The street was wide enough for two running tunnels side by side, but
not for two station tunnels.

One possible reason for such a swap would be to equalize running times
in the two directions. The line was built with the stations on humps
in the profile, to assist acceleration and braking, but this wouldn't
be possible for the lower track at a two-level station. By making the
eastbound lower at one station and the westbound at the other, the
effect is equalized. But this is only my guess.

"Brad" wrote:
I think it's the cathedral and the burial sites surrounding it - I
think the folks who built the original Great Central London Railway
stacked the tunnels because the authorities at St. Paul were afraid
that the digging might disturb or destroy the various gravesites
around the cathedral.

As I said, all of my sources say the vertical arrangement was only due
to the street width. And by the way, there's no "Great" in the CLR's
name. None of the Underground-predecessor companies had Great in its
name, except for the ones that were named in part after the Great
Northern Railway.
--
Mark Brader "I like to think of [this] as self-explanatory."
Toronto "I hope *I* think of [it] that way."
-- Donald Westlake: "Trust Me On This"

My text in this article is in the public domain.