Train Fitness has a built in rest alarm, which will count you down from a pre-set time, and notify you when your rest time is up. Use this feature to stay on track in your workouts and keep disciplined rest time.
You can enable audio alerts for the rest timer to be notified through your earphones when you have 15 seconds and 5 seconds remaining in your rest time. Enable this through your settings either from your Train Fitness app on your iPhone or Apple Watch.