For anyone who's interested, I've uploaded a set of photos I took
recently of the underground railway that links Heathrow T5 and its two
satellite terminals. The rubber-tyred railway isn't visible to anyone
not using the satellites.

The two satellites are mainly used by long-haul, wide-body flights
(most European flights use the main terminal):

https://www.flickr.com/photos/reclin...57679819076761