Zabaikalsk is the final stop on the Russian side of the Russia-China border on the Trans-Manchurian route from Moscow to Beijing. Trains from Russian stations, Irkutsk, Ulan Ude and Chita stop regularly at Zabaikalsk with a weekly service from Chinese stations, Manzhouli (Mandschuria), Harbin and Beijing. The service from Irkutsk decouples from from a Irkutsk-Kharbarosk at Chita waiting six hours before coupling with the Chita-Zabaikalsk service. It is possible to look arround Chita during this break or stay on the carriage in the shunting yards. Toilets are closed for the full 6 hours with the station a 15 min walk away.
The route Chita-Zabaikalsk is serviced daily. If you get off in Zabaikalsk, you need to take a bus to cross the boarder to Manzhouli but you have to buy the train ticket at Manzhouli at the spot(no pre-booking service).
http://www.poezda.net/ can be used to check train schedules.
The bus station is located 800 m from the train station. After leaving the train, head along the platform in the direction from which the train came. Take the footbridge out of the station then follow the station fence in the direction the train was heading (ie China). Look out for the flashing lights on the giant welcome gate.
Expect no one in Zabaikalsk bus station or officials on either side of this border to speak English.
Several busses run daily between Zabaikalsk and Manzhouli, China. The last bus from Zabaikalsk bus station (located near the train station) leaves 15:10 so it might be a good idea to arrive there earlier as Zabaikalsk is not propably a place where you are interested to stay overnight.
Busses arrive at the city centre of Manzhouli.
Bus tickets for this route are 300 rubles plus 100 rubles for baggage. An additional fee of 200 rubles is paid on the Chinese side of the border.
Zabaikalsk is almost more a village than a city. The main reason you are here is propably because you are either going to cross a border to China or you've come from China to Russia.
There most likely isn't much to do for a tourist here.