I used a small torch on a Weaver boiler and it worked great until I got it too hot and a lot of details fell off, so be careful with the heat. I haven't started putting it back together yet, but I'm going to try a 120 watt soldering iron and hope that works.
For diecast boilers (like Lionel) do you just file if off then carefully patch the hole? The dome I want to remove is cast in, but if I remove it there will be a large hole in the top of the boiler.