The Ejs Einstein's Train and Tunnel model displays the famous thought experiment from special relativity where a train enters a tunnel as seen from two points of view. In one case the train is seen in the reference frame of the tunnel, while in the other case the train is seen in its reference frame. The relative velocity and the rest length of the tunnel can be changed via a text boxes. You can modify this simulation if you have Ejs installed by right-clicking within the plot and selecting "Open Ejs Model" from the pop-up menu item.
Einsteins Train and Tunnel Model
Quelle: Open Source Physics