After examining the possiblities through Murmansk and having them invalidated by plane sightings, I now think that Narvik Norway is a high probability of being leg 5 pit stop. There is a direct 168km train between Kiruna and Narvik:
Ofotbanen, called Malmbanan in Sweden, is the single-track railway line that connects Kiruna (in Sweden) to Narvik. The route of 168 Km, entirely north of the Polar Circle, goes along a fantastic landscape of mountains and finally along the Ofotfjord. The first part of the journey after leaving Kiruna is in a typical Nordic tundra landscape, then, after Gallivare station, the train starts to go uphill into a mountainous region. The line crosses the Abisko National Park and there is a stop at the very small station of Abisko that serves the tourist centre. When he train reaches Vassijaure you are at the most northern station of Sweden. The train crosses the border between Sweden and Norway at Riksagransen, which is also the point where the railway reaches its highest altitude. There is no customs to mark the border and the only thing that marks the change is the fact that the train now goes downhill.
I have described here just the trip from Kiruna to Narvik, but the train journey I made was actually all the way from Stockholm, on a train that has of course sleeping cars and restaurant. It departs from Stockholm in the afternoon, to arrive in Narvik in the afternoon of the next day.
See Lapland map below for why this makes sense geographically. Narvik is NNW of KIruna.
Here are the weekday schedule from Harstad/Narvik Airport (EVE) to Oslo:
Dep. To Flight Airline
06:45 08:30 Oslo SK4079 SAS
06:50 08:30 Oslo DY233 Norwegian Air Shuttle
11:05 12:45 Oslo SK4083/DY2 SAS/Norwegian Air Shuttle
17:15 18:55 Oslo DY229 Norwegian Air shuttle
17:55 19:40 Oslo SK4095 SAS
19:35 21:15 Oslo DY237 Norwegian Air Shuttle
Only the ones to Oslo matter, as they can easily connect to Stockholm and from there to St. Petersburg. Flights from Oslo to St. Petersburg connect through Helsinki, Stockholm, Hamburg or Copenhagen. We are looking for a flight through Stockholm having the right time parameters.
The possibilities are:
OSL ARN SK864 1100 1200
ARN LED SK732 1310 1635
Either SK4079 or DY233 get teams to Oslo in time for the connection on to Stockholm and then St. Petersburg.