As the name suggests, the mod swaps Dexter DeShawn's head with Thomas the Tank Engine. Modder camtino has shown off their 'Thomas DeShawn' creation on Nexus Mods. Resident Evil 2 famously swapped Mr X with blue locomotive, while Kevin Brooks' Skyrim mod to replaced dragons with Thomas and got him in trouble with Mattel.
Thomas the Tank Engine mods are nothing new to video games.