What you should know about the trains from Belarus to Russia:
Because of the disagreements between the governments of Russia and Belarus, entry to the Russian Federation from the territory of the Republic of Belarus by any means of transport other than airplanes is possible only for citizens of these countries. Even passengers with Russian entry visa are not allowed to enter Russia from the territory of the Republic of Belarus by train (or any other mean of ground transport). Even if you manage to enter Russia from Belarus by train (often there is no control), it can be considered an illegal border crossing, as it`s impossible to put an entry stamp in the passport on the open border.
What you should know about visa-free entry to Belarus:
Foreign nationals may visit any place in Belarus within 30 days of their visa-free stay. However, they must arrive and leave the country only through Minsk National Airport. The visa-free entry rules do not extend to any other international border checkpoints or airports of the country. Belarus visa-free travel rules for foreign visitors do not apply to people flying to Belarus from or to Russia (these flights are considered internal and are not subject to border control).