I think ballast compaction would prolong the service life of the ballast. A ballast bed with an excess porosity within due to insufficient ballast particle contact will likely be unstable. Acting loads on to the ballast bed, may mobilize this unstable contact condition and start moving the individual ballast particles within. The accompanying interface frictional stresses among the mutually interacting ballast particles along with possible dynamic impacts occurring due to gap closure may induce levels of stress that can change the particle size distribution of the ballast bed.

Ballast compaction should prolong ballast life so long as ballast is not damaged in the process. Compact ballast would be expected to form a strong aggregate matrix to resist applied loads. Uncompact ballast would strain excessively under load, causing particle abrasion and breakage. The compaction process can be achieved under the right conditions under train traffic. However, a separate compaction process generating a more uniform level of compaction would be better for ballast longevity. Differences in the track structure and train passage will lead to variable compaction along the track compared to a controlled process and some sites may not develop adequate compaction under traffic. These variations will result in differential track support and could start the formation of differential strain, settlement and, ultimately, ballast degradation.

Ted had the answer. Also, compaction will help to achieve better particle interlocking which improves ballast strength and stability.

With the cases I tried before, the frequency of compaction will have influency on performance of the ballast.

Ballast life is affects due to many reasons and Compaction is one of them,

Railway traffic continuously increasing, ballasted tracks need more efficient maintenance processes. Lines with long welded rails which are prone to buckling during heat waves require stabilization before being fully operational. Stabilization is performed either naturally using regular traffic at penalizing lower speeds, dynamic stabilization of sleepers or alternatively crib compaction.