Idea: Bedrock rails.
Not sure what it would do, actually. ... Hmm, an industrial-coil powered minecart?
perhaps be able to be placed over air for a limited distance?
maybe require more bedrock rails on land to support... so to cross a 20 block wide hole, you need an additional 6 rails, 3 on each side to support it. or perhaps a solid, refined block as bedding (not cobble/dirt/gravel/sand/etc) under the rails on each side of the bridge
or something like that. if concrete wasn't reactor-craft only, I would suggest that using concrete as a railbed allows carts to go faster on normal rails, and is a requirement for the air-bridging ability of bedrock rails.
bedrock powered rails would be more powerful.
ofc, all the bedrock rail types would be indestructible (which brings back the air-bridging thing, or have them never despawn if they get uprooted) unless the transition to other rail types got taken out. (so don't uproot if the rails on either side of the new hole are bedrock, but if there is a regular rail there, uproot the whole segment)
just tossing ideas out here.