This trail is part of the Bernina Express
ride which is marked with the standard red and yellow waymarking signs as trail number 673. This segment is one of the highlights of the Bernina Express
Hopefully, you checked your brakes before the ride, because from the Alp Grüm station you'll lose 400 m (1300 ft) in less than two miles on your way to Cavaglia station.
This segment is a gravel singletrack from about 1-4 feet wide, so watch your speed because the trail surface might slide out from under you if you lock your brakes. Also, there are some metal water bars along the trail that you may want to jump over. Hitting one of these at speed might cause an instant flat tire, not that I am speaking from personal experience :) .
You'll also pass over the train tracks a couple of times, so watch for trains.