The mod replaces the Tree Sentinel's steed with the children's TV show locomotive, making the early, optional boss substantially less intimidating in the process. That's unusual because I normally find Thomas's blank, wide-eyed face scarier than whatever he's replacing, but the absurdity of having a big angry man on his back cancels out any potential fear.
The Tree Sentinel Thomas Mod by creator skymoon can be downloaded from Nexus Mods. The downside is that it's not as easy to install as some other Elden Ring mods, which only require dropping files into a /mods folder. To make Thomas work, you need to use a UXM - a tool not currently available via the Nexus - to unpack the game's files and overwrite the Thomas model in the right place.
If you do go to the trouble, it has another feature: it also replaces the player's own leaping horse with a Thomas model. You can see that in action in the video from the mod's creator above.
And if a mod can't satisfy your thirst for train-based horror, you might want to keep an eye on Choo-Choo Charles. It's an entire game, still in development, about being hunted by a blood-thisty spider-train with a grinning, bloodsoaked face. The Fat Controller would not approve.