No legitimate store listing (Google Play, Amazon Appstore) shows version 1.3.7.9 . The most recent official legacy version is around 1.2 or 1.4. 7.9 likely does not exist as an official increment.
Approximately 12.09 MB to 12.6 MB (main APK installer) trainz simulator 1.3 7.9 apk
You might be wondering: "Why don't I just download this from Google Play?" No legitimate store listing (Google Play, Amazon Appstore)