When coming from the Bernina pass Poschiavo is the first big town on the way to Tirano in Italy. As the train is coming down a steep track the brakes make a lot of noise and smell too.
The train station has a big kiosk which sells drinks and snacks too. Plus the post office is in there too.
The main mean of transportation is the Rhaetian Railway. The trains run from St. Moritz in the Upper Engadine over the Bernina Pass down to Poschiavo, pass the lake and after Brusio the famous circular viaduct before crossing the border into Italy and ending in Tirano. The elevation difference is remarkable: Start is in St. Moritz at roughly 1800 m, highest point is the Bernina Pass at 2256 m, the end in Tirano is roughly at 440 m.
Especially between the Bernina Pass and Poschiavo hairpin curves, tunnels, bridges and covered galleries had to be built for the train climbing down the steep slopes. Thus the train is not exactly an Express train - though the marketing claims so.
Regional trains run almost hourly and stop at each little place (sometimes upon demand, so don't hesitate to press that button which is to find in the cars), Express trains run a couple of times daily and stop only at the major places - they need reservation and cost a surcharge. I personally love the regional trains that also have open-air cars attached in July/August between St. Moritz and Poschiavo.
Postal buses go to the places where to which the train doesn't go. Since recently there are direct buses from Pontresina via the Bernina Pass road (that takes a different route than the train) to Poshiavo/Le Prese. These run quite often in the mornings and afternoons but not hourly.
Smaller buses serve the small villages/hamlets in the side valleys: From Sfazu at the Bernina Pass road into Val da Camp, from Brusio up to Viano and another to Selvaplana on the eastern side of the lake (the train runs along the western shore). These buses run infrequently. The schedules are co-ordinated with train and Postal bus, though and convenient for hikers IME. Please note that reservation is obligatory for these smaller buses! Also, the bus into Val da Camp costs a surcharge (Alpine Ticket).
The phone number for reservations is to find on www.postauto.ch