Getting the access rights to allow a fleet of units to get the extra 10mph is a pain in the botty.
you have to prove the safety case now, that extra 10mph will mean higher impact speeds etc, regearing of the motors - in the case of 317,319 the g315bz traction motor, do they need extra cooling etc, how much power will be dragged out the overheads to get them to this speed how long will they be able to stay at 110 etc
the 110mph that the LM 350's have allows them to slot in between the pendos on the old lines when on crewe jobs..it took alot of time, research by LM, Hitatchi and the LM front seaters ,it has paid off.
for the extra 10mph 317 - 321 u need to be able to prove crashworthiness for all in the unit, motors can handle the extra power etc, braking, vehicle ride. etc etc
all of the old BR traincrews know that the original 317/1 fleet were more than capable of tonning up, im unsure of the class 317/1, the 319 & 321 speed records im sure some of the old school front seaters will know the records and will i suspect tell you what they could get out of some of units..
Even 310s could be taken off the clock, they had a design speed of 75mph, easily could do 90's and lets face it all of us who remember them especially on LMR, some of the old girls could do it..others could only handle 75..
same with dmmus, ive been at least 100 in some flying meccano,
in the same way HSTS could easily outrun 125 and are governed, all fleets can exceed their built and current design speeds, it is not done now...as if anything happens, then speeding will always become a factor in any incident.
but in todays culture you dont go over the exact speed of the unit..
its not just about regearing and lobbing bigger pads on the brakes.