So I re-created my old world called Tachanka with version 1.16.1 to experience the nether update and went into a Mineshaft, fought off a few mobs in the Mineshaft and when I found a zombie just standing and looking at me. It couldn't walk to me because of the "railroads". Basically, the AIs in this game is treating the railroads as a block that they cannot walk to or over to. I've tested in creative of all behavior type of mobs; passive, neutral, and hostile mobs and they cannot walk over any of the railroads in the game. Though, when hit over the railroad they will walk over it like normal if they are passive/neutral mobs. For hostile, they are able to walk over the railroad after getting damaged by the player or pushed/knocked onto/over the railroad. Flying mobs like the bat are not affected. However, I did not test if this also affects the aquatic mob's AIs.
All of the screenshots attached to this report are tested mobs that are not able to get to the player or the villager.