wrote:

I rode the train from Lewisham to Elmer's End back in Sept 2001.
I recall the right-of-way being rather wide, like 3 or 4 tracks,
but my memory is fuzzy...

I don't think that is the case - the Mid Kent line was built with two
tracks and has never been any more than two tracks, nor was land taken
for more than that.

I wonder if perhaps you might be thinking of the line from Lewisham to
Grove Park, which is wider (being the main line) and might well have
the possibility of accommodating a DLR extension (going on to take over
the line to Bromley North). This was mooted when the Lewisham extension
was first discussed in the late 1980s.