This I feel would be quite a simple yet beneficial feature to add for 8th gen players who may feel they are missing out on content they have purchased from previous routes, where they can add locos from other routes which were cut from the timetables due to memory issues. I think that the 9th gen layers should be off by default for all routes which have memory issues, but then having the option to add them on for 8th gen would be so much better to allow more variety in services. For example, on the Brighton Mainline, lets say I wanted to do a 166 service, I could just enable it in the new layers feature and then have just that layer on along with the usual 8th gen content provided. That would mean that no mass amount of services from South Eastern would cause memory issues, and yet would provide so much more gameplay for those who have bought content from elsewhere.
