So, the train route via the border Khasan/Tumangan to Pyongyang is still not officially available for tourists. But there is a 2nd train from Moscow to Pyongyang. It goes via China.
Or you can take a train from Moscow to Vladivostok and then continue by plane to North Korea.
Now there is a travel agency which offers North Korea trips in combination with the Transsiberian railways: Please have a look at http://koreakonsult.com/index_eng.html.
The agency is located in Sweden, I know them very well ;-)
I can really recommend them and Julia - she is the owner of the company - is indeed a very nice and helpful person. She can organize everything for you to get from Europe to North Korea via Russia and China - except the route via Tumangan.
As I wrote, we bought tickets for the Tumangan-route by ourselves and didn't tell the travel agency or KITC (the state tourist agency in North Korea) in advance about our real route into North Korea. It was a dangerous experiment and KITC and the travel agency were quite upset after that. And they also took precautions to prevent, that such happens again: Now the tickets to Pyongyang have to be categorically organized by the travel agency, individual ticket buying is no longer accepted by KITC, otherwise they would cancel the trip (and the visa).
That's why I do not recommend to repeat what we did!
TUMANGAN - really totally impossible?
Well, not really. Just recently I found out that it is theoretically possible to enter North Korea at Tumangan as a Western tourist. But it is not possible to take the train from Tumangan to Pyongyang, which we took.
There are Russian travel agencies offering trips to the "Rajin-Sonbong Special Economic Zone" in the very north of North Korea, adjacent to Russia. They are aimed at Russians, but I found out, that also other foreign tourists are accepted on this tours. These tours go to Tumangan by train (usually starting at Khabarovsk or Vladivostok), where the tourists are picked up by buses and brought to a sea-resort at Rajin.
Here are some links to Russian travel agencies offering trips to Rajin:
I'm not sure whether they will be able to reply you in English. It might be necessary, that you speak some Russian to communicate with them and get the things organized.
However, now also KoryoTours (they are aimed at Western tourists, which makes it easier, if you don't speak Russian) offer now trips to Rajin, but starting from China. However, according to their website it's at least possible to leave via Russia and the Tumangan/Khasan border crossing.
Meet the North Koreans in Russia!
Or if you just want to get a special experience you can reapeat what I did in 2006: Just take the North Korean waggon for a domestic trip (like Moscow - Irkutsk) inside Russia. This is totally legal and it's likely that you will be the only foreigner among North Koreans on board of this waggon. If your're interested, just contact me (firstname.lastname@example.org).