Most of us actually do just file. The trick is to use a good full sized 8" to 10" or so mill file. The same good large file used to file the points and the frog rails.
The key is to support the rail well - I use a grooved plate that holds the rail sideways. It's a bit of extra work - slower than a belt, but for me it allows better control.
First timers tend to not file enough rather than too much. After one or two efforts - you'll get the hang of things just fine. Don't be afraid to just get started and be willing to redo things at first.
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-ed mccamey-
COSLAR RR -
http://www.coslar.us/NMRA Standards and Conformance Department
PROTO & FINE Scale Coordinator
I estimate I have about 5 pounds of coupler springs somewhere in the vicinity of my workbench.