Current Behaviour:

Rails are considered as a walkable block, if an NPC (only tested with villagers) stands on a half slab next to the rail, but not as a walkable block if the NPC stands on the same level as the block.

In the video, you can see that the villager walks over the train tracks from the platform made from half slabs, but cannot get back out. In the video, the mod "Mini Hud" [1] was used to highlight the pathfinding. This does not affect the path finding to my knowledge and the same behaviour was observed without this mod loaded.

Expected Behaviour:

Rails to be considered as non-walkable block in the case an NPC wants to walk from an adjacent half slab over the rail, as it is when standing normally next to the rail.

[1] https://www.curseforge.com/minecraft/mc-mods/minihud