Yes, Russia does have a (small) border with North Korea. Russia controls the Pacific coast to a bit south of Vladivostok, the terminus of the Trans Siberian railway, and home of russia’s Pacific fleet.
The border between the countries is a river, which is crossed by a railroad bridge. Because North Korea uses standard gauge tracks to be compatible with China, the Russian gauge trains cannot be used without a change of gauge.
So far this looks like a defensive deployment to prevent a flood of refugees in the event of an active military confrontation between the US and North Korea.