On Jun 10, 1:57*pm, John B wrote:

So what's the latest (alleged) fault that's required the 377/5s and
378s to be taken out of service all about, then...?

Found this by chance (another FCC webchat today it seems):

Q91
When do you expect to have the Class 377/5 units back in service and
why were they so unceremoniously withdrawn with such short notice?

A91
We stood down the 6 377/5 units last Friday due to a problem with the
earth return brushes which was found on unit 377 502 on routine
maintenance. We then examined 377 505 and found a similar problem. We
therefore stopped the fleet for examination and rectification. There
are 14 sets of return brushes on class 377/5 and not more than 30%
have been found defective on any unit. However we do not want these to
deteriate further and the situation to go unchecked so the
manufacturer, Bombardier, have been working on a solution. This
solution has started to be introduced today and we expect to have
trains back in service at the rate of two per day.