What is a "tamper" ?

This isn't a London-specific transport question, but seeing as I'm
subbed to uk.t.l but not uk.transport, and there's a sort-of-London
connection, I hope you'll forgive me :)

Picture at http://www.flickr.com/photos/85667196@N00/228439736/ or a
larger size at http://www.flickr.com/photo_zoom.gne?id=228439736&size=l

I saw that sitting at Slough station[1] yesterday, and as I had no idea
what it was I snapped a picture. A quick search of the web has turned
up the fact that it's an Amey Tamper ... but despite further searches I
haven't been able to find out what a Tamper unit actually does. So if
someone could let me know I'd be very grateful indeed.

Cheers!
Iain

[1] The "London-ish" connection is the fact that it was at Slough.
Sorry, very tenuous I know!
.