The ticket is still valid if you change to another metro line, but if you change to bus or tram another ticket must be purchased. Trams are suprisingly able to keep to time and if the timetable lists a particular interval the chances are that it will arrive as scheduled. Occassionally trams do get diverted across a different route and can catch out the unsuspecting traveller. Again there will be no ticket collector. Simply have your ticket stamped by the machine just inside the doors.
Trams drive on every larger road and generally they stop at the metro lines.
After midnight the night-trams drive about every 40th minute. 91 is the historical tram, which passes by almost every historical attraction in Prague, except for the castle. The round-trip lasts about 1 hour and costs 10 Kc for adults and 5 Kc for children Persons with luggage or prams are advised to enter at the back of the tram. Buses are used where there are no trams and follow the same ticket procedure as trams. Tickets of all type entitle the holder to transport free of charge one of the items subjected to the above mentioned charge.