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.  

Ejs Einstein's Train and Tunnel model was created using the Easy Java Simulations (Ejs) modeling tool.  It is distributed as a ready-to-run (compiled) Java archive.  Double clicking the ejs_ehu_sr_train.jar file will run the program if Java is installed.  Ejs is a part of the Open Source Physics Project and is designed to make it easier to access, modify, and generate computer models.  Additional Ejs models for special relativity are available.  They can be found by searching ComPADRE for Open Source Physics, OSP, or Ejs.

Please note that this resource requires at least version 1.5 of Java (JRE).