Java versions 7 and 8 for macOS include AppleScript support, but do not advertise it to Java applications. This JAR advertises that support to Java applications running on macOS under Java 1.7 or 1.8.
Java version 9 is expected to drop AppleScript support entirely, so this JAR provides no benefits on that version of Java.
You can add AppleScript support to JMRI by entering the following command in a Terminal.app window:
bash <( curl -sSL https://jmri.github.io/applescript/install.sh )
You can also install manually.