I tried the dremel and it worked pretty good, most recently I picked up a 4" side grinder from Northern Tool for $10 with a few wheels, and Man...what a difference in speed of cut and you can trim both rails at once if needed. The cutting wheels are cheap and last Much longer than the dremel!
Train-Li sells a tool to hold track together to make cut "Square"...I bought one when I bought their bender.
As stated above I use a Dermmel tool with a cutoff wheel, works fine for me. Manfred (Lotsasteam) has a micro chopsaw with a metal cutting blade that he picked up at Harbor Freight, does an awesome job.
Another vote for the rotozip tool. I used a dremel for years and had a problem with the cutting wheels being too small to get all the way through the track. The rotozip goes through track like butter. It is also great for cutting limestone rocks which I am blessed with in profusion.