The tram was originally in Fallout 3, the broken steel expansion, not NV. The tram hat was used because of limitations in the engine. I assume the obsidian Devs just copied the existing solution for NV. Also more fun, it wasn't an NPC, it was you, the player character and the tram wasn't a hat it was a technically piece of shoulder armour.


For the unaware, the entirety of FO:NV development could be considered "crunch time" because the studio, Obsidian, got royally screwed on the contract to make the game instead of the main Bethesda studio(s).
The result is that the game originally had some plan for vehicles, most of which never made it past concept, but never got implemented. So, in order to make a moving Tram to ride into vegas from the south, if you somehow manage to complete an obscure and somewhat buggy sidequest to stop it from getting blown up, they just made the Tram an NPC which physically runs along the track. It works surprisingly well.
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