The rail-mobile ICBMs were prohibited under earlier versions of the U.S.-Russia START treaties. However, the 2010 New START treaty did not prohibit rail-mobile basing of missiles, and Moscow is taking advantage of the omission.
Obama is expected to announced soon that he will seek a new round of talks with Russia aimed at cutting U.S. nuclear forces even further than the 1,550 deployed warheads under the 2020 New START treaty.