A motor is driving the wheels which means that the wheels, connecting rods and pistons are all in motion as if steamed up - hence the blurry bits.
ARG_Flickr
CC BY 2.0
<a rel="nofollow" class="external free" href="https://www.flickr.com/photos/38986305@N06/8513091305/">https://www.flickr.com/photos/38986305@N06/8513091305/</a>