I always carry a small power bank, its not big enough for a full charge, but it will get me out of trouble, make a call or send a message. On a related note a lot of trains and even buses have USB charging sockets, so I always try and keep my phone 'topped up' when out for the day. One thing that did surprise was yesterday on a 331, quite a new design had power sockets but no USB sockets
no phone signal (common in rural areas)
Once the ticket is downloaded to the device the Northern app doesnt need a phone signal, I just activate once the train is close by
phone running updates when inspector comes round
I set my phone to manual updates, its not just ticketing that is a problem, I want updates to take place when at home and connected to a solid wifi signal, not when out and about with sketchy mobile signals. I do the same with my laptop. I can see this might be a problem for less tech savvy people.