The only route I've experienced that phenomenon on (the grid lines) is WCMLN and then only on certain platforms, Motherwell being one.
I may have imagined it, but I seem to recall reading that it is, ironically, the "invisible platform" that is used for defining the area that passengers wander about on, but as I say, this might be me misremembering
It shouldn't show up, but I don't know that your system has anything to do with it when it does. My PC is much older and slightly less capable, but sometimes it does this.
Sorry to not be a huge help, hopefully a more knowledgeable route builder will be along to clarify what's going on here.
Rik
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