[IR] Default 3rd/4th Rail

This pack for Immersive Railroading adds new trackstyles with British styled 3rd rail and 4th rail tracks, using the basemod assets.

This pack is licensed under CC-BY-NC-SA-4.0 because that is the license used for the models in basemod IR.

Credit for the original model goes to the Immersive Railroading team (github page here), the original model can be found here with the texture here (both links go to specific pages in the IR assets github)