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If you are spending time in the Volga region, you might be coming from/going to places like Cheboksary, Ulyanovsk, or Samara.  Between Cheboksary and Kazan the bus is definitely easier, as with the train you must go to Kanash and then change trains for Kazan there.  South along the Volga, there is only one train a day, six days a week (not Saturday).