I've recently made a custom model for the powered rail using multipart to define each state of the block with a model, but I encountered a bug where the model does not apply multipart when it is not on a transparent block.

Step to reproduce:

1. Use Minecraft 1.17+
2. Make a resource pack
3. Place the powered_rail.json into /blockstates/
4. Unzip the content of rail.zip into /model/block/rail
5. Place the rail_special texture in /textures/block/
6. Reload the resource pack
7. Place a powered rail on a glass block & on a stone block (they need to have the same tags)
8. See: