A LNER Azuma service departed at 0027 from Finsbury Park on the 14th July and run via the Northern City Line and Cannonbury Curve to the North London Line. Afterwards it proceeded to run to just past Hackney Wick and use the tracks west of Stratford to join the Great Eastern to Liverpool Street. It arrived at 1am.
This was to test emergency diversion in case of significant disruption or planned engineering works towards King's Cross.
Why not use the Graham road curve? Is it unsuitable for an Azuma?
It used to be said that it was restricted to class 313 units. As they are ~20m vehicles with a tight loading gauged, and the Azuma is 26m vehicles with a fuller loading gauge, there could be some issues there!
Graham Road curve is complexicated. IIRC whatever length limits that still exist (or ever existed in the first place) are not to do with car lengths, and more to do with EMU only under traction only and linked to a short track circuit on the spur itself between Navarino Rd Jn and Graham Rd Jn. Something to do with one track circuit and traction immunisation on either side. Or something of that ilk.
There was certainly no limit of 313 only - the current NR Anglia Sectional Appendix shows 313 315 317 319 321 322 345 357 378 permitted; ok some of those are gone now, the SA takes a while to catch up on detail like that. The route was routinely used for Ilford < > Hornsey empty EMU movements. It also had light engine 86s between Anglia and WCML.
SA also shows 170 and 222 permitted but no other diesel unit; there are no 80x of any flavour listed - but that might be part of the reason for testing.
When I did the Liverpool Street Leapfrog tour in 1992, they put some restriction on it: the 33 was only supposed to pull not push over that route, so the tour had to be back planned to ensure it was the right way round. On the day, one extra reversal was inserted to get round a points failure, and we ended up pushing anyway.
All esoteric detail none of which has anything to do with LU; nor does the entire thread ?