Rail Watch uses data from ATCS monitoring servers to do interesting things.
Railroad control and sensing data is collected and decoded from trackside equipment (MCPs) via wireless monitoring equipment operated by a community of interested persons. This server collects all the data and presents it in various forms, charts, maps, and MCP signals.
The charts attempt to decode the data and show it in time vs location format, and by viewing these you can see detailed train movement data. Sometimes the algorithms can't figure out which directions things are actually moving so you might see arrows pointing the wrong ways.
From the charts you can reach the detailed MCP lists for a particular division. Clicking on an MCP will show its database entry along with recent control and indication bits that have been received.
When GPS location data is available for railroad control points, a map is drawn showing recent movements. These are color coded circles from red (most recent) to blue and with reducing size. Small blue circles indicate no recent activity, and small white circles indicate no data collected.
Many thanks to those who run ATCS monitoring sites and servers, and to those who collect, analyze and maintain the MCP databases.