I had an cool idea for an app: When I need a connecting service from the tube to a mainline, my phone should alert me whether I need to run for the train or if

I had an cool idea for an app: When I need a connecting service from the tube to a mainline, my phone should alert me whether I need to run for the train or if I have time for a coffee when I get close to the station. At the moment, I travel at irregular times and have to bring paper timetables around everywhere. I might even have a go at building this, if I have enough time.