It is simple but brilliant. It tells you the times of the next trains northbound and southbound from any Dart station (and much more I'm sure but that is what I use it for).
It was written by Stephen McBride apparently and full credit to him. I am using the free version at the moment but I like it so much that I'm going to upgrade, not because I need the additional functionality (not even sure what that is!) but simply because I find it so useful.
I have even used it when standing IN a Dart station (it's more dependable than the screens!)
I see on Google Play (what an awful new name for the app store) that some people have problems with the app. I have a problem when I look at the RSS feed having received a notification but apart from that it generally works just fine.
Highly recommended.