I've been away from home for a day or so, so only just seen this thread, but I was actually on Northallerton station for a couple of hours yerterday afternoon, and could just see the front of the affected HST under the bridge about a mile to the north of Northallerton station.
For about 90 minutes, both up and down lines were closed, and then down trains started moving very slowly passed the failed train on the up. Trains to and from Middlesborough continued as these lines were not affected. Also a TPE from Newcastle and an EC HST came off the Middlesborough line - these must have been the ones diverted via Eaglescliffe.
Then about 1540, the affected HST crawled very slowly through Northallerton - the front power car being ominously silent - I noticed too the front coach was empty - cleared presumably as a safety precaution? As has been reported, it was switched to the up slow just south of the station, and I'm not surprised to learn it was caped at York.
Then, when I was expecting things to start to get back to normal, the next southbound EC, which was 91-hauled, came up the down line to the crossover at the southern end of the station, before reverting to the up fast. Were they checking the up power line, or was the power still switched-off?
A little later, three XC Voyagers powered south on the up line in quick succession, before the class 67 Thunderbird arrived at the south end of the station, reversed and crossed-over to the up slow, following the affected HST. I thought it strange that it was following the train, and if the HST reached York before being cancelled, I wonder if the 67 movement was entirely seperate from the afternoon's events.
Unfortunately, I then had to leave.