This is the question that generally haunts most of the RAC and WL ticket holders. But as a matter of fact, RAC and WL positions always tend to improve. How far the RAC and WL position will improve depend on certain factors such as festive periods, availability of train, how much days in advance were the tickets booked and so on. However there are three reasons as to why the RAC and WL positions always tend to improve
Low cancellation charges
One of the prime reason for the improvement in the RAC and WL positions is the fact that the charges for cancellation of train tickets are comparatively quite less. As a result several people always have an option of cancelling their train tickets on various occasions which include change in their travel plan, etc.
Every bookable train has different quotas for the different groups which include railways or other government employees, handicap people, military and so on. So when the tickets for these quotas do not get sold, then they are used for adjusting the RAC and WL positions.
In case when there are available berths then the ticket holders can be given a free upgrade in a higher class. The important thing to remember here is that this system of free upgrade of the passenger is decided in the later stages and a passenger can be upgraded to only two places above the class that he/she has paid for. In case of a busy train the chances of free upgrade is very minimal.