you can build an underground railway or underground road, but you can't build underground one-way railway. This option is not even available, there's just no button for this. You can only build a bridge with one-way railway (over the road for example) but you can't build a tunnel.
Take a look
1. Railway - you can build it above or under the surface

2. One-way railway - you can build it above, but you can't build it under the surface

I think there is no reason to cut off this option
