The Trainline is a company which provides ticket purchase services to a huge variety of train operators in the UK. The main one I care about is South West Trains.
They've just released an update to their app which saved my bacon this morning. At my train station, there isn't always someone in the ticket office first thing in the morning. I already have a season ticket for my main journey, but if I want to go to London, I need to buy an extension ticket. You can only do this online or at a ticket office, but not at a ticket machine.
The Trainline app on iOS helped.
1) There are no longer any fees for on-day purchases
2) Tickets can be picked up after just 5 minutes of purchase. Sometimes SWT themselves will say you have to wait 2 hours to pick up your ticket.
Thank you Trainline!