This KOMATSU excavator that appears to be able to propel itself along the rails was used by London Underground
whilst relaying track on the Central Line near Chigwell in March 2006.
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Photo from James & Martin's Picture Collection Copyright 'Jampics' m.hawkes7@ntlworld.com 
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British and Foreign Railway Pictures <A HREF="http://jampics.fotopic.net/"> Jampics Fotopic.</a>