Check out this "Separating Blocks" thread from earlier this week...
http://www.trains.com/trccs/forums/1286196/ShowPost.aspx
Conemaugh Road & Traction circa 1956
Cement it into place, then use a #11 x-acto blade to carve the extra material away.
It's not hard, just fiddly.
When cutting the gap in N scale track for isolation, what's the best way to fill it before painting. Everything I've read says to cut a piece of styrene of the proper thickness to fill the gap, glue it in the gap and then file it to the shape of the track. File it?? Are they kidding? How do you get in there with a file to shape it to the rail? Rounded file? How do you Nscale guys do it?