The "tractor' is of course Stephenson's Rocket which, apart from having a copper firebox and a few rods missing, is the original steam locomotive that won the Rainhill Trials in 1829 and ushered in the railway age. Chelsea is the "Explainer" who was giving hourly talks on the history of the exhibit.
Uploaded to Flickr by ARG_Flickr on 19 December 2018
Photo © ARG_Flickr, 19 December 2018. Licensed for reuse under this Creative Commons licence