Are there any maintenance issues regarding the railway crossing the Hexham Swamp?
Are there any other places with similar issues?
How does Hexham Swamp compare to "Mud Holes" on say the Albury-Melbourne line?
Since you are using the present tense, I assume you mean the four track main line from Sandgate to Tarro and not the former J&A Brown line to Richmond Main.
I haven't seen any evidence of mud holes on this section, although I've only seen it in detail at points with relatively easy access.
However, while there are, or were mudhole problems on the vaguely similar Bredalbane plains section on the South, many of the worst mud hole problems on the South are in cuttings as in Gunning just north of the station and just south of Mulwaree Ponds in Goulburn.