Chances are that this is original. I wouldn't worry about removing the picture rail. If you need to re-plaster and you keep it in situ you will lose some of the profile behind the plaster. It will be very difficult to create a successful plaster job with it in place. Remove it carefully, strip it if you can be used be bothered (it is a lovely shape) and refit. Alternatively get a timber merchant to cnc copy the profile or a fibrous plaster company to mold copy the rail and produce in plaster.
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