More trains are on their way for Crossrail and these ones are yellow.
Testing is currently underway in Continental Europe to allow a Linsinger MG31 rail milling train and two multi-purpose engineering trains to be delivered to London ahead of the Elizabeth Line opening in December. The yellow plant will be used during the night to monitor and maintain the Crossrail routes.
The 48-metre-long rail milling train is the first of its kind to be used in the UK and is able to scan rails using electromagnetic crack detection, looking for any defects. If it identifies any issues with the track, it can mill the surface of the rail to remove defects and cracks, reducing wear on the new Class 345 train wheels and tracks. Metal chips will be collected in a container on the train for recycling as scrap metal.