As I model CP Rail, I use the CN Diesel Bell - Bill, our host, recorded it from a CN SD40 if memory serves. It's got a nice tone and you can hear the mechanism whacking the dinger against the bell - just like the CP GP38-2s that run behind my house (when they aren't converted over to that god-awful E-Bell they have now a days.)
I've also used the Bell Bronze file for a S3. Slightly faster ring rate, beautiful ring echo when you finish the ringing.
I'd download a couple of different files and load them to your loco. Then set them up one at a time and see which one you like the best.