Mrs Frustrated Old Bag is a diesel locomotive on Sodor Island.
Nothing is known about Mrs Frustrated Old Bag's past, before she ended up working on the North Western Railway.
When she started working on Sodor, she was given the job of pulling Roly. At one point, she played Christmas music for three weeks in a row. She then asked Roly what he wanted for Christmas, while Roly thought about choking her.