Isolating a section of already-ballasted track?

[Steamtrains]

I've often had to cut laid track (only a couple of times already ballasted) using an Atlas saw and a block rail gauge to keep the cut nice and square. After cutting I use a small file to get any burrs off. If your ballast is real stone, I recommend you scrape it off at the cutting point. Rocks and saws don't mix....
I've tried using the Dremel but it really leaves a very "wide" gap that needs to be filled.

The nippers are great for cutting rail to be laid. You get nice accurate cuts, and require almost no dressing with a file.

Patience helps whatever method you choose....