I recently bought the Caltrain MP15DC Diesel Switcher for TSW 2 Peninsula Corridor and am really enjoying it. I uninstalled TSW 2020 but will it be available on that too?
It will only be available in TSW2020 if you bought the version for TSW2020 (which then becomes available in TSW2 through preserved collection). If you bought the TSW2 version, it won't be backward compatible.
Unless you are certain you won’t ever go back to TSW, I’d suggest buying the dlc for TSW as every route and locomotives except the north east corridor route and associated locomotives don’t come over to TSW2.