Pocket Trains will sync game data between iOS devices using iCloud and between Android devices using Google Games. Cross platform syncing is not supported (iOS to Android and Android to iOS).
For iOS Devices ensure the devices you are trying to sync are logged into the same iCloud account from the device's main Settings menu under iCloud. If you already have a game in progress on both devices, this will sync up the game with the most XP and the older game progress will be lost.
For Android Devices, go into the Pocket Trains Options menu and login to the same Google+ account on both devices. If you already have a game in progress on both devices, this will sync up the game with the most XP and the older game progress will be lost.
To begin syncing, the Gift Code shown at the bottom of the Options menu needs to match up on both devices. If there was an error detecting a previous game or you've reset the game from the Options menu on one device a new code will be assigned. To get the codes to match again, delete the app from the device with the game you want to overwrite, then earn some XP on the first device (with the game you want to keep and sync), close the app and reinstall on the secondary device. Deleting the app will delete all game data on that device so don't delete from the device with the game you are trying to sync.
If you've previously disabled syncing in the Pocket Trians Options menu you will need to re-enable this on both devices to resume syncing.
How to Disable Syncing:
If you want to have separate Pocket Trains games on devices sharing an iCloud or Google Play account, you can turn these options off in the Pocket Trains Options menu on both devices to prevent syncing.