Thomas put the Blues on the board first at 14:40 of the first period, scoring from the top of the paint on a second effort tap-in. Just 39 seconds later, Connor Bleackley doubled the Blues' lead to 2-0 when he picked up and deposited the loose change from a Wade Megan shot.
After a scoreless middle frame, the Caps showed a bit more fire in the third, especially after St. Louis forward Dmitrij Jaskin challenged Caps winger Tom Wilson to a fight, the former taking exception to the latter's hit on Thomas in front of the Blues' bench early in the final period.
Thomas scored his second of the night on a St. Louis power play at 12:22 of the third, seconds after the Caps couldn't quite make good on an excellent shorthanded scoring opportunity.
Kostin tallied at 16:28 to close out the scoring.