Yep, this has been a legitimate "talking point" since Rosie was re-designed because fans would rather recycle old talking points rather than try and find something new to discuss. And since I haven't produced a post in a while, and this is a fairly easy topic to get my thoughts out on, here's why I think the complaints are stupid.
Defenders of the new lettering say it's "Railway Series accurate" which, let's be honest, isn't true. Sure, rolling stock may have had NW lettering (the Spiteful Brake Van being the most notable example), but the only engines that did were Pip and Emma, and that was solely due to the fact they pulled the Wild Nor Wester to London.
That's not to say no engines are bare: Mavis has the Ffarquhar Quarry Company on her sideplates while Bill and Ben have SCC on their tanks to represent the China Clay Company. But they're the only ones really, and they're technically not on the NWR's books.
Because of this, and Rosie's re-design, I actually want the engines to gain the NWR lettering at some point. First off, it's three letters that represent the railway they're working on, for crying out loud. Why shouldn't they have it?
Second, do fans realise that the numbering system is actually NWR specific? Some engines keep theirs for the sake of heritage (probably), but the main 7, Donald and Douglas were all given new numbers when they were bought/saved, so the NWR branding would just enforce that idea. They don't even need letters, just a logo to represent the railway.
Third,the North Western has been mentioned a few times by name in recent years (TAB and Bubbling Boilers being the two most prominent examples), and the lettering addition would be a nice way to capitalise on that.
Basically, the whole thing is a complete non-issue. If they want to add lettering to every engine, fine. If it's only for Rosie, fine - if a bit odd. The major thing to take away is that nothing would change regardless. It'd still be a show about fictional engines on a fictional railway on a fictional island having adventure.
The only difference? The show's staff would actually acknowledge that the main standard gauge railway has its own name. That's it. Fans need to learn to pick their battles and grievances. There are a few issues with the show that stop it reaching higher than it could do; this sure as hell ain't one of them.
Oh, and to those saying it's not right for other fans to do this sort of stuff? Leave them be. It's their headcanon for their series. Stop trying to ruin their fun.