The Rail, Maritime and Transport Union has defended today's mass walkout, saying it goes far beyond the question of how opens and closes the train doors.
The RMT has staged a 24-hour walkout on the Northern and Merseyrail network, in addition to the Southern line - which has witnessed repeated disruption over the past year. Around a million people have been affected.
The dispute has thus far centred on plans for driver-only-operated trains, which would remove the task of opening and closing train doors from the guards and make drivers responsible.
However RMT boss Mick Cash says that this goes further - suggesting it's all about ensuring commuters have a second safety-critical person on board the trains who can help passengers in an emergency.
Cash pointed to the recent collapse of a wall at Liverpool Lime Street station, when RMT staff were praised for their work in evacuating passengers, as an example of the important work done by guards.
He suggested that, by removing the second safety-critical person, the train service would be worse. But Julia responded by pointing out that a worse service is better than no service at all.