A reverted gear train is very similar to a compound gear train. They are both used when there is only a small space between the input and output shafts and large changes in speed or power are needed.
There are two major differences between compound and reverted gear trains. First, the input and output shafts of a reverted train must be on the same axis (in a straight line with one another). Second, the distance between the centers of the two gears in each pair must be the same.