You have 2 go to The MPDL power station near the scrapyard, or NW of bethesada ruins, Or
almost straight north of megaton, jus follow the power lines, its inside the schematics on
how to build it is in little building, on a desk, its a piece blue paper, after that u jus
need to find the parts and a work bench

as my good friend shadowstep said , it is a custom-weapon , and you will need schematics
and the proper parts to make one at a workbench , you repair level will determine the
rifle's condition.

There is also another way to get it , during the normal course of the game you have random
encounters , if you are lucky enough (which has nothing to do with your luck in the game)
you will encounter Laszlo Radford , he is a hostile wastelander and will open fire on you
on sight , he carries the railway rifle which you can loot from him after you have killed
him.