If you have an explanation, I'd like to see it, and the source.

The most detailed explanation I've seen is this one from a very
hardcore expert:
http://districtdave.proboards39.com/...e=1#1163453810

The gist is the tunnels were built in stages with no real plan, and it
was very late on that a decision was taken to make the tunnel with the
shallowest gradient up to King William Street the northbound one,
which also happened to be the one on the right crossing the Thames.

Thanks, that's an excellent explanation and it completely makes sense.
It includes many details that I have not seen in any book -- and I'd
still like to know what source they came from, so I can read it!
--
Mark Brader | "What a strange field. Studying beings instead of mathematics.
Toronto | Could lead to recursive problems in logic."
| -- Robert L. Forward (The Flight of the Dragonfly)