Richard J. wrote:

Staying overnight in a house in Southgate, some way from the
underground section of the tunnel (which runs under playing fields of
the local college), it is still possible to hear the trains rumble
through if you listen carefully enough late evening or early morning.
Given the location of the house, which was nearly on the High Street,
this is quite surprising.

You're probably only about 500 metres from one of the open sections of line
either side of the Southgate tunnel, in which case it's not surprising that
you can hear trains. That seems to me a more likely explanation than
ground-borne noise from the tunnel.

Yes, 500 metres from both ends, in fact.

However, the noise is a constant getting-louder-rumble,
getting-quieter-rumble. It's not what you'd expect to hear from an open
section (and I should know, because I live near one).

The tube is probably running about 250 metres away underground at this
location, judging by the map.