The rail cart is first introduced at the end of the training portion of Half-Life. Most are found in the tunnels of the decommissioned underground rail system of Black Mesa's Materials Transport. They are powered by electricity. The player can activate and manually operate a train from their control panel, controlling its speed and direction of travel in forward or reverse. The path of a train can be changed by rotating the switches at rail junctions.
Behind the scenes
According to John Guthrie, the idea for the rail cart came from level Rabid Transit from Duke Nukem 3D which featured a subway. Guthrie cited the level as one of his favorites and always wanted to have the opportunity to drive the train. The yellow train from the multiplayer map Subtransit was originally created for the abandoned rail system section of the chapter Blast Pit. However, it was replaced in the final game.
Subtransit rail cart.