Paul Scott wrote:

Rebuilding the 'sub surface railway' track and formation to mainline
standards throughout, so that the S stock will ride properly when
delivered.

Mainline track standards are much lower than LUL track standards: when you
have a short track and lots of tiny trains, you make the track perfect and
save money and space by having crap suspension on your trains, whereas NR
have lots of track and few trains so they build their track out of discarded
goalposts and buy proper suspension for the trains. Hence the "Buckaroo
Line" where LUL trains run on NR track.

But it's a bit worrying if the new trains need better track than the old
ones did.