Rail grinding is often done to restore rail surface that had become corrugated and causing the horrible roaring rails noise. Straight after grinding there is usually a different and quieter higher pitch sound due to the relatively fine grinding marks. That normally reduces after a period of weeks. If things have gone well, the rail corrugation stays away but often it returns later if track stiffness and other parameters haven't really changed much.
The old rail grinding car converted from Standard Stock was a rather crude scheme compared to modern rail grinders. It just had grinding blocks pressed down on to the rails so the amount removed was probably minimal and wouldn't restore profile and level very well. Modern machines use proper rotating grinders with much better control of the amount removed and profile produced. There is evidence that current 'modern' Tube rolling stock propulsion equipments influence rail corrugation adversely. This and other threads have also discussed the effects of different track forms and resilient baseplates etc.
There seems to be an implication, here, that rail grinding reduces noise.
And yet I'm sure I've seen it suggested that noisy sections have been caused by rail grinding.
Is it a bit hit and miss on the noise front?
Rail grinding introduces it's own, different, noises*** that diminish within a few days; at least on main lines it tends to be similar to a whirring sound.
Rail grinding may reduce existing rail noise conditions in certain circumstances, but, if so, that noise is a symptom of a rail problem being addressed, not the problem itself. Yes grinding may eliminate what was a noisy section, and the result is tangible to travellers, but what it has really done is rectify whatever the underlying fault was causing noise to appear as a symptom.
*** on main line again; my residence is close-ish, but not adjacent, too the main lines through Luton; can always tell these days when the rails are fresh ground as today's 700s wheel-rail noise is near identical to yesterday's loud 319s noisy traction motor \ pinion noise; in the transition period from 319s to 700s I could not tell the difference between the two. After a week or two the newly introduced rail noise diminishes.
One of the main reasons - if not the principal reason - for grinding and one not mentioned so far is reduction in RCF; at least on the main lines this one is possibly the biggest driver behind all these grinding machines now operated by contractors for NR. It was RCF that caused the 2000 Hatfield accident.
I don't know how much of an issue RCF is with LU, but rails are rails, wheels are wheels, and the principles of steel wheel on steel rail applies to LU same as anywhere else.
It is a technical subject, and apart from me not understanding a good deal of it myself, TBH I can't really post much more.
One thing though is that them across the pond seemed to have cracked (pun intended) the issue ahead of the rest; UK main line I think had one not-that-large train-machine (the Speno) - until Hatfield - after which they proliferated here. One of the US suppliers is Loram, and, ok, accepting this is from a concern that wishes to sell you something, it is quite a good explanation of modern grinding:
EDIT site is defying me posting URL maybe it don't like some of the ascii; going to find another way to link to it
not sure if that URL will survive; if it don't then suggest paste it into google as a search item and it'll probably find the right one.
One fact too is that fatigue of any sort is proportional to load cycles, and LU (pre covid) is ever more busier, more trains, longer trains, longer heavy loads peaks, in turn then fatigue load cycles go up that induces rail issues in shorter durations. Even trains braking - and LU brake rates are quite high - impose stress loads on rails - while your braking mechanism may be friction on the wheel or pad or electric on the motor axle, that braking effort is still transmitted from wheel to rail (it don't work otherwise!). It all adds up.