With the OLE I'm aware that the tensioning is insufficient for multiple pantographs at 140mph, but what causes the issues with the track itself, and why wasn't that a problem with BR's running at 140mph? Simply track wear becoming more extreme with more frequent running at the speed?
For points, it is not permitted to have discontinuity in the rail above 125mph (as at all conventional switches and crossings), so they must be replaced with Swing nose crossings, as seen on high speed lines across the world. This was actually done at Ledbury Junction for the WCML upgrade, and then later undone!
For plain line, it comes down to sleeper spacing, which in turn is about track stability, particularly though any areas that involve a lateral force ie all curves. For high tonnage 125mph lines you need 28 or sometimes 30 sleepers per ‘length’, and if speeds are being increased to 125 then you need to assess any areas that don’t have this spacing to demonstrate the track will stay where it is supposed to. Going to 140mph makes this more of a challenge.
The good news is that 28/30 per length has been standard spacing for about 20 years now for renewals, but I can guarantee there will be many areas that are 26 or even 24 per length. And the work isn’t as simple as chucking an extra couple of sleepers in every so often!
There will also be alignment issues, particularly transitions into / out of curves.
BR didn’t run at 140mph in regular passenger service, at least not officially. Special dispensation was granted for each move.