s8 stock performance upgrade

This modification increases the initial acceleration everywhere Finchley Road and North of there. South of there already has this initial acceleration. This gives a few seconds advantage on most interstation runs, which gives better recovery margins or more timetable flexibility, prior to the full performance with ATC.
The mod now is possible because power supply upgrades were done in preparation for S stock and ATC so the old low acceleration that A stock had 'flag up' is no longer necessary to keep within power supply capability. It's only the intial acceleration so the trains remain compatible with the existing signalling.

Line speed maximum 60mph, only in certain sections.

Still missing the A stock

No, not yet. But Regenerative braking can raise the voltage near to 790V. The nominal 750V supply is still to come, to support ATC performance in some areas.
The present tweak to S8 trains only affects about 0-17mph

How likely is that to happen out of interest ? The most I've ever gone is 2 mph over and then only for a few seconds.

That sounds very unlikely for the sort of gradients that occur on railway lines, given that the motors will drive the train pretty fast on the level.

It is my understanding that (subject to any governer) the maximum speed or "balancing speed" of any powered vehicle is the speed at which the power delivered to the wheels equals the drag of the train (rolling resistance, aerodynamic, gravity). In an electric motor the power that can be delivered is limited by the back emf, (the electricity generated in the motor as it tries to be a dynamo) which increases with motor speed - thus there comes a point where the back EMF generated by the motor balances the force trying to turn the motor. This is akin to terminal velocity foe a skydiver - the faster you fall, the greater the wind resistance until you reach a speed at which the wind resistance balances the force of gravity and you stop accelerating.

Just as a skydiver uses a parachute to increase wind resistance (drag) and thus reduce his terminal velocity to something survivable, the magnitude of the drag of the train, and therefor the balancing speed, will depend on a number of factors, notably aerodynamics (greater wind resistance in tunnels), rolling resistance (generally greater for heavier trains) and gravity. Thus the balancing speed will depend on all these factors, and unless governed, an electric train should go faster downhill than up.

(*The back EMF can be reduced by reducing the field strength, hence the "weak field" settings which improve maximum speed at the expense of low-speed acceleration.

www.vnerr.com/training/intro_loco_app.pdf

For an unpowered (coasting) train to accelerate down a gradient, the gradient need only be steep enough for gravity to overcome rolling resistance. This is true whether it is accelerating from rest or from 60mph. Rolling resistance may be greater at 60mph than at walking pace, but there are plenty of railway gradients steep enough for this to happen.

This is why, I understand, trains are allowed to go up Shap faster than they are allowed to go down - it talks longer to stop going downhill because you are fighting gravity instead of working with it.

Something to do with the motors acting as a speed limiters once theyve reached theyre maximum limit, coast and then the motors are not stopping you from going faster.

An S stock Rolling through Willseden Green N/B at 50mph will go through Neasden at 60mph, one can guess what speed one going through Willesden at 60mph could reach.

That's possibly why there have been few replies to this thread from actual drivers.
Even behind usernames it's difficult to put into words describing 'experiments' that might have been undertaken on new rolling Stock.

I think it's interesting to compare theory with reality. In my experience all trains roll (coast) differently and it doesn't take very much of a downhill gradient at all to accelerate through 60 mph.