In message , Richard J.
writes

TfL journey planner maps

You have to devise a route that uses the tube line in question.

Ah, thanks. It is interesting to see the route of the Piccadilly at
South Ken if you enter something like Onslow Square to Knightsbridge.

Interesting! My Bartholemew's of 1956 (3" to the mile) shows a left
bend after South Ken, then a straight north-easterly run (not following
the road pattern) to a second left-hander to align with Brompton Road,
then a right-hander following the curve of Brompton Road by the Oratory.
If yours shows the correct left-right-left-right sequence, which roads
does it run under?

No, sorry, it doesn't. This has come up before, as I recall.

I think the second bend (following Brompton Road) is correct but the
section before that is wrong. I have had a look at numerous early
20th-century maps, and most follow Bartholomew. Some of the Bacon maps
show a tightly curved exit from the station, so that the line runs
beneath and across Thurloe Square - but I think that is also wrong,
since the position shown would produce tightly curved platforms.

I suspect the most likely route is that shown by the about Journey
Planner map, except that the Piccadilly lies directly beneath the
District line rather than a little to the south of it.

The reason is not only that the shape of the reverse curves seems right,
but also the fact that the triangular site above the first curve (i.e.
south of South Terrace) was a riding school at the end of the 19th
century. That would doubtless have made it much easier and cheaper to
obtain a wayleave for this one section of line that was not directly
beneath a road or railway, rather than having to pay owners for
tunnelling beneath multiple private properties. (The riding school is
marked as "works" on later maps, after the Piccadilly was built).

--
Paul Terry