Belarus double-entry transit visa, number of days: days there or days valid?

Be aware that your route might not be possible at all, at least not legally. Foreigners are not allowed to cross the border between Belarus and Russia by land even if they hold visas for both countries. The reason for this is the lack of border checkpoints between the two countries. With no border formalities on the train, you, technically, enter Russia illegally. In theory, you might enter Russia and return to Belarus without any Russian official checking your documents.

In practice, experiences may be different and there are people having done this trip without problems. The information I received from the Belarusian embassy (May 2019) is the following:

Taking practice into account there are no problems for traveling from Russia to Belarus with international train after entering Russia via international checkpoint of the Russian Federation. Problems arise when foreign citizens try to cross border of the Russian Federation from Belarus by land coach (car, bus, train). Russian border guards in these cases deny exiting Russia and sent to Russia–Latvia border.

Nevertheless, it is suggested to check this with Consular section of the Embassy of Russian Federation when submitting documents for Russian visa.

I suggest clarifying the current situation with your closest Belarusian embassy.

As to your question, the Belarusian MFA defines a transit visa on their homepage like this:

transit visa (type B) with the right of single, double or multiple transit through the territory of the Republic of Belarus within 2 days;

To me, that sounds like you would need two transit visas in which case a short-term (type C) visa might be the better choice.

However, I strongly suggest reaching out to the Belarusian embassy with your exact situation and itinerary. They can probably provide more detailled (and up-to-date) answers.

The company Pul Express specialises in invitations and visas for travel to Russia, Belarus, and Kazakhstan. They have advised me:

tragen sie bitte als Einreise Datum von Russischesvisum minus 2 tage und als Ausreise plus 2 Tage.

which means

Please write for entry the date of the Russian visa minus two days and as exit the date of the Russian visa plus two days.

So if a Russia visa is valid from 3 September to 17 September, then the Belarusian double entry transit visa should be valid from 1 September to 19 September (I guess to be on the safe side.

I'm aware that formally, travellers cannot take the train from Belarus to Russia, but according to several online sources, direct trains from the EU are allowed and neither seat61 nor realrussia has any report of any traveller being taken off this train.