Today’s invention is another way for tracked military vehicles to maintain mobility when their tracks are damaged.
A number of individual, three-wheel drive units is provided. Each has its own motor and track. These tracks bear on the larger regular tank track which is still powered by the drive wheels in the usual way.

When the main track is broken, the small units can independently keep the vehicle going.
Wow…see this
http://www.core77.com/blog/object_culture/modified_subaru_is_an_awesome_way_to_get_around_in_the_snow_15569.asp