With a tip of the cap to Greater Greater Washington, we thought you might enjoy this useful web application, which shows where Metrorail trains are in real time.

The application, created and hosted by The Unofficial D.C. Transit Guide, uses WMATA’s developer API to pull data (the same that you see on Metrorail station public information displays) and crunches the numbers to come up with an approximate location. You can determine the direction the trains are going by looking at the letter on each train’s pin — the terminus the train is heading to is represented by the first letter of its name, like “V” for a Vienna-bound Orange Line train.

It’s not perfect — but it is a great example of the useful tools which can be created when WMATA allows people access to its data.