In this occasionally updated PBase album, I have placed some documentation shots showing 'tips and tricks' to removing the shells on various N scale locomotives.
As always, work carefully, just because this worked for me, doesn't mean a manufacturer hasn't changed something or that your model has a modification that I'm not aware of. Use at your own risk. That having been said, I trust this is helpful.
See the step by step instructions in the descriptions in each photograph.