Beltran supplied a solo shot in the sixth inning and a long, two-run homer in the eighth for the defending world champion Cardinals, who forged a 1-1 tie in their best-of-five NLDS series.
David Descalso also went deep for the Cards, who banged out eight extra-base hits.
Lance Lynn (1-0) earned the win after taking over from starter Jamie Garcia in the third inning despite giving up two runs on three hits in three frames.
Washington's Jordan Zimmerman (0-1) was tagged for five runs on seven hits in a three-inning effort.
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