Fun way to see the town, the tram takes you through quite a long route. The bad thing it's usually packed with tourists and it's tiring to be inside without any personal space.
I would definitely recommend starting at one end of tram route. This gives you the best chance of getting the seat/location of your choice. Even waiting at one of the starting points you aren't guaranteed to get on the first tram that comes by, but it is worth the wait to see all the sites and crawl up, down and all around Lisbon.
If you can do not go during the summer peak season. Siberia can get very hot and there are lots of inserts (especially around Lake Baikal). Go in late September for the fall colors, warm days and cool nights. If you want to see the stunning snow and ice scenes, go late December to January.
Accommodations are spartan by Western standards, make sure to get a first or second class ticket. You might want to do this trip with friends since the sleeper cars hold 2-4 people. Make sure to confirm sleeping births reservations for each section of the trip (and for the correct days), otherwise you might be out of luck...