1. Make sure your device supports Augmented Reality (AR). To see a list of devices which support AR, follow this link : ARCore Supported Devices

You must also make sure that you are using a version equal to or higher than Android 7.0 (Nougat).

2. Allow the app to use the camera. If you had denied access to the camera when first launching the app, you can grant access by following these steps through your device's settings:

Open the Settings on your device

Go to the application manager

Find the Thomas Minis icon and tap on it

Find the Permissions menu. You should now see the permissions for the camera, make sure it is toggled on.

3. Open the App. After you completed the tutorial, tap on any train track. If you tap on an empty board, place at least one track segment on it. You will now find this icon in the upper right corner:

or this icon:

Tap on it. You will now see a grid. Align the grid on a flat surface and tap the screen. 

Your track now appears in augmented reality! You can resize the track by tapping the lower left side buttons.