It is possible to reach the Valle dei Ratti in Three North line train departing from Milan (Central Station), with destination Sondrio-Tirano and change at Colico for connection to Chiavenna. For then arrive and get off at Verceia and from there continue to walk/bike.
Arriving by car
By car you can reach the Valle dei Ratti from the suth through the SS 36 (Milano-Lecco-Colico) and Como through the SS 340 Regina.
While from the north from Passo dello Spluga (closed in winter) and the Passo del Maloja through the SS 37.