It is not running already for many years. It used to be an everyday train, but since the fall of USSR traffic obviously declined, then the train was substituted for a carriage Riga-Truskavets via Lviv, but the carriage stopped running like four or even five years ago, so at the moment there are no direct rail connection at all.
And even more, nowadays there are only 2 permanent international trains running from Riga, one to Moscow and one to Sankt-Petersburg (
http://www.ldz.lv/?object_id=4422). So the timetable you found probably just some kind a leftover of model schedule left in 'Ekspress' system. It is there, but it is not used. Similar thing happens with many non running trains.
p.s. btw, there are almost every year a rumor that previously running Riga-Gomel train will be "reincarnated" (Riga-Truskavets carriage, as far we remember, used to run with it), but it always stayed a rumor, unfortunately.
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