According to a report in Kommersant, Russia conducted a test of the RS-26 missile on March 18, 2015. The missile was launched from the Kapustin Yar test site and the warheads (or warhead) successfully reached the target at the Sary-Shagan site.
Kommersant reports that the missile will soon be formally accepted for service and that its deployment will begin in 2016 (confirming the earlier reports). It also said that first RS-26 missiles will be deployed in Irkutsk, but that appears to be a reference to an old story.
UPDATE: Interfax is quoting a source in the ministry of defense as saying that RS-26 differs from RS-24 in that it has fewer stages and a shorter range. If true, this appears to confirm that RS-26 is a two-stage missile based on RS-24 very much in the way SS-20 was a two-stage version of Temp-2S.