I don't want to set myself up as an expert, but I think we had better be careful here, or an urban myth may be in the making.
From what I've read, heard and assimilated, in the Sevenoaks accident a River class tank (2-6-4T?) was involved running at speed, that is 65-70 mph. These locomotives had got a name for rocking around on SR track, and the SR abandonned using express tank locomotives as a result of this accident, rebuilding all Rivers into 2-6-0s (Us or Ns, I forget which).
However, a River class loco was indeed tested on the GN, and found to ride perfectly ok.
It was ascertained that the shingle used by the SR (possibly only on the former SECR sections, and taken, I believe, from around Dungeness-New Romney area) did not have the properties of good granite ballast (what a suprise!) and the SR forthwith stopped using shingle and steadily replaced it.
The Hither Green accident was a very different affair, caused by cracking of the railhead across the fishplate bolt holes. I can't remember if it was put down to nose-suspended traction motors, but it was generally deemed that track on the modern (for 1957) railway was subject to a far higher degree of pounding that it had been previously.
Hither Green was a turning point in many ways - it resulted in a determined effort to do away with jointed track, indeed any breaks in the rail, on main-line routes, and, as one lecturer on BR course said once (something like) it led to the emergence of the DMU-EMU railway, with the loads from traction equipment spread across the train, avoiding locomotives with heavy axle loads where possible.
I am happy to be corrected, but I do not think there was any allegation that SR track was somehow sub-standard in general from the Hither Green accident.