Most landscape places around here that sell them will cut them for you if you buy them. If not, a drill and sawzall should do the trick (drill pilot holes along the path of the cut and at the angle you want to cut and use the sawzall to work it small sections at a time between the pilot holes).
Sounds like too much work.... I'd just put the whole tie into a hole that's dug so that the railroad tie slanted down.