From wikipedia:
"While there is no single standard that applies worldwide, new lines in excess of 250 kilometres per hour (160 miles per hour) and existing lines in excess of 200 kilometres per hour (120 miles per hour) are widely considered to be high-speed, with some extending the definition to include lower speeds in areas for which these speeds still represent significant improvements"

To be fair, we don't know what the design speed of this upgraded line is. According to one railway site here in the UK:
"The reports are that the train was doing 81, which fits the speed governing at 79mph (with a small delta) which is the maximum for single manning where there isn't PTC (the American train protection system)."

PTC is fitted to this line, though not yet in use.

The same experts estimate that the overturning speed might be nearly twice the speed limit, to allow for late braking or poor adhesion. So a train going at 50-60mph might not derail. But it depends on all sorts of factors.