On Mon, 14 Apr 2008 14:40:04 -0700 (PDT), Mr Thant
wrote:

On 14 Apr, 22:23, "Railsigns.co.uk" wrote:
There's an elevated Limit of Shunt signal at the Richmond end of the
Up platform (no. GB507). As you say, there are no routes reading up to
it. It's fitted with a trainstop and it's there to prevent trains
setting back wrong road to Richmond without authority.

And you can see a photo of it on page 15 of this 5 MB PDF:
http://www.irse.org.hk/chinese/newsl...ary%202007.pdf

And also a comprehensive explanation.

Including that the offending move (as done on at least two previous
occasions) being prevented is a train returning to Richmond "wrong
line" in error rather than proceeding past the crossover at the London
end of the station and using that crossover to return to Richmond on
the Down Line.