This is an End of Train Device (EOT or sometimes commonly known as a Flashing Rear End Device FRED). It is designed for you to be able to add your own electronics in to it. It also is designed to fit in to a closed coupler and be held upright so the flashing can be seen from behind you.
There are 2 files to this design; and obj and stl. The obj file is the whole unit with the lid off, the stl file has the front sliced off of the unit. I recommend using the stl file unless you would like to pull out off the support structure from the inside by hand. Feel free to modify this, add on/off switches, etc. If you come up with a different one please let me know as I might like to print it for myself too. I hope you enjoy!
Hatchbox PLA or ABS
If you print the whole object you'll need support everywhere, otherwise you'll only need support on part of it.