Trail is a web-based app, which means you don't need to download or install anything.
You can access it in a browser like any normal website, but pinning Trail to the home screen of your device (iPad, tablet, phone etc.) gives you all the benefits of a native app with all the flexibility of a web app.
Add to Home Screen on iPhone and iPads
Open Safari and go to web.trailapp.com
Sign in if you're not already signed in
Click on the address bar and double-check the address reads "https://web.trailapp.com/trail#/" without anything after the ".com/trail/#/"
Tap on the share icon at the bottom of the screen
Then click "Add to Home Screen"
Add to Home Screen on Android:
Open Safari and go to web.trailapp.com
Sign in if you're not already signed in
Click on the address bar and double-check the address reads "https://web.trailapp.com/trail#/" without anything after the ".com/trail/#/"
Tap on the menu button at the top right of the screen
Then click "Add to Home Screen"
