I suspect everyone who has purchased and installed the MTH WiFi app has discovered the speed issue. What happens is you accidentally bump the upper end of the speedometer without knowing it and the train accelerates to maximum speed.
Open the app on the device you are using to run trains and click on the settings key. Scroll down through the menu and you’ll find a maximum speed setting. The max speed is automatically set at 120 scale mph. Move the setting button to the left to establish your desired maximum speed. You’ll need to do this for each MTH PS2 or 3 engine you run using the app. My suggestion would be to do this immediately whenever you purchase and add a new engine to the app.
Curt