This affects resource packs that provide a custom model for different rail blocks. To visualize culling I've made a small helper indicator, where a letter disappears if block is culling in this direction.

Straight rail blocks cull with full blocks but not with other rails:

Same with ascending rails and corner rails:

This happens with all 4 types of rails. Additionally, rails should also cull with matching sides of different types:

Below is an example of my rail model whose endings disappear faced against full blocks but not against each other:

One important note is only the corresponding sides should cull, as highlighted here in green. Sides marked with red shouldn't disappear.

I've also attached a resource pack that adds the culling indicator I used to make these screenshots (It's the same I used to report MC-178889 hence it affects chain and iron bars as well).