LEGO Half-Life Rail Cart from "Apprehension" Episode
This is the rail cart from the first Half-Life game, from the episode "Apprehension". It's that high speed rail cart that you get after launching the satellite, and then crash it into a water pit in the first 10 seconds as it gets derailed. It also appears in the intro as it crosses the path of the tram.
Photo gallery with comparison screenshots.
Though this might not be the most iconic thing from Half-Life, I'm kinda proud of this build, because it uses a bunch of fun building techniques. You can also repurpose it as a bumper car if you want :D
This build is fully functional as a train, you can push it along the track. It even works without the tiles, although that makes it a bit bumpy and noisy, but it's a nice cheap alternative. This works, because the rails are offset to the side by half a brick, making them impossible to connect with the cart's base plate.
For a functioning rail switch, see my previous Rail Cart MOC.