I added diagonal rails.

It was tough to find an entity to base them on. My requirements were:
1. Must support direction
2. Must support render_layer

I am using assembling-machine for now, but I would be happy to change that if someone finds a better solution. Everything seems to have side effects.

Things I have tried with their side effects:

assembling-machine
- Assembling machine stats in tooltip.
- Fluid arrows on mouseover and in blueprints.

fluid-turret
- Turret stats in tooltip.
- High priortiy biter target.

straight-rail
- collision_box can't be used to detect collisions while building it.

I could drop the render_layer requirement if I do a bunch of control.lua scripting to transform entities as they are built. I may end up doing that.

If the fluid arrows on assemblers bother you set off_when_no_fluid_recipe = true.

Furnaces might be an alternative to hide the no recipe line. I think they also inherit render_layer.

Fluid arrows are required, I am using fluid_box pipe_picture to get my render_layer.

So off_when_no_fluid_recipe = true removes the entire fluid box not only pipe connectors and arrows?
Good to know.

What do you need direction for? If it is only to show different graphics, https://wiki.factorio.com/Prototype/SimpleEntityWithForce or https://wiki.factorio.com/Prototype/SimpleEntity should do the trick. Note that simple entities interact weirdly with rotation, they can't really be rotated properly, it is advised to turn that off with the prototype flag.

Every diagonal bridge has a north/south configuration and an east/west configuration. They rotate perfectly in blueprints, but only if the entity supports direction.