Two thoughts.
The first would be quite controversial, as there's effectively 2tph into Waterloo, it should be possible to run 1 train in 3 to Exeter rather than the normal timetable of 1 in 2.
You could mitigate some of that loss in frequency by running longer trains and/or running a Honiton/Exeter shuttle.
The other thought is, could the fix be to include some retaining features. The thing being that if the embankments are flatter then:
Any rain during summer is more likely to be absorbed rather than run off the steep banks
There would be more soil towards the edges of the embankments, this would mean more water to dry out before impacting the structural area of the embankment
This would also mean that if redoubling was to be done in the future the earthworks were already more stable
Linked to this, the banks would be more stable during the winter and less prone to slippage.
As the retaining wouldn't be structural (i.e. sat beyond the 45 degree angle of repose) they could be fairly basic. For some of the lower embankments this could be little more than a paving slab laid near vertical to add 400mm at the edge of the embankment (this would allow an embankment which is 1.6m high with 1 in 3 side slopes to have 1 in 4 side slopes - and I suspect that most embankments aren't even as flat as 1 in 3).
Whilst flattening side slopes doesn't sound a significant improvement, even small improvements can be quite helpful in their ability to improve the stability of them.
The first would be quite controversial, as there's effectively 2tph into Waterloo, it should be possible to run 1 train in 3 to Exeter rather than the normal timetable of 1 in 2.
You could mitigate some of that loss in frequency by running longer trains and/or running a Honiton/Exeter shuttle.
The other thought is, could the fix be to include some retaining features. The thing being that if the embankments are flatter then:
Any rain during summer is more likely to be absorbed rather than run off the steep banks
There would be more soil towards the edges of the embankments, this would mean more water to dry out before impacting the structural area of the embankment
This would also mean that if redoubling was to be done in the future the earthworks were already more stable
Linked to this, the banks would be more stable during the winter and less prone to slippage.
As the retaining wouldn't be structural (i.e. sat beyond the 45 degree angle of repose) they could be fairly basic. For some of the lower embankments this could be little more than a paving slab laid near vertical to add 400mm at the edge of the embankment (this would allow an embankment which is 1.6m high with 1 in 3 side slopes to have 1 in 4 side slopes - and I suspect that most embankments aren't even as flat as 1 in 3).
Whilst flattening side slopes doesn't sound a significant improvement, even small improvements can be quite helpful in their ability to improve the stability of them.