After dropping down into the final train car there are a couple sliding doors at the back end. Opening them up and looking to the left there is a red box on the wall. Inside that (in pitch darkness) is a glowing green button you can push. Once pushed, it spawns the titular glowing lambda symbol in the center of the train car.

Couldn't figure out a way to interact with it, so I figured I'd reach out the community to see if anyone else has happened upon it.