There's an 'introduction' on the BBC web site:
http://news.bbc.co.uk/1/hi/england/london/8620188.stm
Consists of their Transport Correspondent explaining what a 378 looks
like. He makes it sound like they are exclusive to the ELL and have
not actually been in use on the Overground for months...
Story includes some political squabbling alleging that Boris has been
trying to claim the credit for opening the line. Nothing new there
then.