Mud holes are over the entire network guys, lets get that clear.
Also, no-one creates mud holes but they don't exactly fix them correctly to prevent them from re-occurring either.
Re-sleepering doesn't prevent mud holes either, it actually can create them.
Major track works when track machinery for Tampering, re-leveling of track can actually create mud holes... especially when forced to work through the rain... Vibration of the sleeper to compact ballast under neath is effective but when raining.. remember ballast drains, if the capping is destroyed underneath, the vibration will shake the mud through the track... and Hello.... new problem..
How there fixed?
They dig them out, dispose of material (Unsure where)
Instead of repairing the capping layer to prevent the mud coming through, they just fill them up completely with ballast.. Seen ARTC do this with my own eyes over the years..
V-Line do the opposite, fix them properly.. Metro 50/50