Toddler benefits most from the bed rail.These are those with greater height, when the join field in spring and mattress. This means that the longer distance models fall.Most toddler bed rails are manufactured to fit between the mattress and box spring, with some variation of L-shape. The actual height of the rail is different, and may be influenced by the depth of the mattress. This is an important point to consider when looking at the size of rail.Before finalizing the bed rail, measure the height of beds, mattresses and depth for extra protection.