This mod adds so called "Bridge Rails" which can, for the added cost of a bit more steel and wood, be placed on water.
This allows you to drive your train over bodies of water without having to fill them in with landfill.
To craft Bridge Rails you have to first complete the necessary research (after Automated Rail Transportation).
I am not very active on here, so please check my linked Github account for any news!
Version 2.0.4 now works with Factorio 0.15!
Special thanks to Github user d3x0r for updating the mod!
This mod was originally released for Factorio 0.11.x, when it was way more useful, because landfill wasn't yet part of vanilla Factorio.