Quite a significant moment this, thanks for sharing!
I've done a bit more research into the Mk3 OLE used on the MML, specifically the single-track cantilevers.
For speeds below 200 km/h (125mph), cantilevers look like this, as seen here at Ampthill.
View attachment 86713
The steady arms on the cantilevers have a
shallow curve, and don't allow for the greater uplift given by pantographs at higher speeds. In addition, the lower horizontal tube requires a nose dropper, whether pull-off (toward mast) or push-off (away from mast), and the steady arms require windstays to prevent them form being unhooked.
For 200 km/h speeds and above,
these cantilevers are used (as seen at Frinkley Lane, ECML).
View attachment 86714
These arms have a
deep curve and a larger bracket separation (around 300mm nominal); the nominal gap between the contact wire and the bottom cantilever tube is also around 150mm larger. Also note the fixings for the steady arms are different; the 'drop down' allows for the 'heel setting' (arm pivot to contact wire) to be adjusted more easily. This feature isn't present in the cantilevers seen at Ampthill (or anywhere else on the 80s-era MML OLE).
Hope this attempts to clear things up a bit!
Edit: The only Mk3 registrations which appear to be 125mph capable are the mechanically independent registrations in Luton Airport Parkway and a half tension length or so north of Elstree & Borehamwood; for the latter, a mid-point anchor portal was installed and all headspans to the northern overlap (+ 1 to the south) were replaced with MIRs from 4-track portals. (The Fasts then have 4 STCs on each road south of that last portal before Elstree & Borehamwood station; admittedly the linespeed here is 110 max.)
Edit 2: Brent Cross West is also getting MIRs of a similar style.
