Railroad Position System
Summary: The Railroad Position System can track the position of transmitter equipped vehicles.
From the JMRI Website:
The RPS System allows you to locate the exact position of locomotives and other rolling stock that have been equipped with transmitter decoders. The system makes up to 10 measurements per second and each reading is accurate to much better than an inch, which makes many imaginative uses possible.
See the JMRI Hardware Support page here for more info.