The resource pack attached offers a complete fix for this issue. Mojang have my full permission to (and are encouraged to) replace the affected vanilla model files with the contents of the resource pack.

Download resource pack:

• 1.17.1: consistent-planes-1.17.1-v1.0.zip

The bug

The "flat" and "curved" rail template models are completely identical, and therefore redundant, since models referring to one template could always just use the other.

How to reproduce

1. Navigate to the game jar's models/block/ directory
2. Open rail_flat.json
3. Open rail_curved.json
   1. Notice how these models have no differences from each other aside from in name

How to fix

The resource pack attached fixes a handful of bugs with plane models (where the plane is attached to a block) in general - as many issues as could be found. This redundancy is one of them - both the "normal" and "curved" rail template models are merged into a single "template_floor_plane" model (which also fixes MC-212626). Lily pads are made to use a tinted version of this model (fixing MC-162803), the shading is removed (fixing MC-198311 and possibly MC-234118), and all planes are made to be distance 0.1 from their supporting blocks (fixing MC-155551 and MC-212256). MC-214700 is also fixed on the side as well due to including a plane the same as those in this ticket.

These template models should also help eliminate any future discrepancies like those detailed and also be of use to resource pack creators and modders who can reference them instead of having to create a new template each time.

A resource pack which only fixes this single rail issue can be attached on request, but the mass fix resource pack is recommended.