I run Trainz on a RAID 0 setup that "Syncs" to another RAID 0 setup exactly the same, so I know Trainz runs fine on a SSD. SSDs on Mac OS do prefer to be formatted APFS and HHDs prefer HFS+. The will "talk" to each other if you mix, so no worries. Is your external SSD formatted APFS or HFS+? Also, as I mentioned above, open Trainz/Trainz Settings/Install/ and change/confirm the Data DB Folder is to path of your DB Folder on the external SSD. I have found that for some reason if you leave the DB folder on the Mac and also on any external, Trainz likes to revert back to its default on MacBook. I thinks its in the code so If I go external or anywhere else, I remove or rename the default DB folder on Mac so Trainz can't find it as long as I'm on external. If you want it and it isn't currently set to your external, be sure to make the change on the path, shut it down and restart to make sure Local Data path change doesn't revert back to Trainz Default. This only needs to be done once. Be sure and backup your current DB folder, put it somewhere safe and if things start going screwy make sure Trainz didn't revert path back to default, unless that's where you want it.