With no commerically viable domestic sources of fossil fuels, there has always been a push in Sweden to go for local sources of energy. A fairly large number of reasonably sized rivers made hydropower an interesting alternative and large installations were made in the early 1900s. The main railways in Sweden were electrified from 1914 and onwards and at the time of WWII, a substantial coverage had been achieved, even by international standards. Today more than 95% of the rail transport effort is done with electric traction.
With the development of modern attack aircraft in the 1930s, the military strategists started to recognize this dependence of fixed installations as a serious vulnerability at a time where the military doctrines was all formed around rail transports. A particularly sensitive part was the power feeding points, where the three-phase power from the main grid was to be converted into low-frequency, single phase traction supply using rotary converters. An air attack or sabotage against such an installation would cause a total disruption of the rail traffic that would take a long time to repair. The rotary converters were proprietary to the rail network and given their relatively small numbers, a destroyed one would therefore be difficult to replace in a wartime situation.
Shortly before WWII it was therefore decided to build a decentralized structure with mobile power converters that could be moved to shelters, spread out all over the country. As the war progressed, the decision was changed to favor blast-proof shelters, typically caved as solid rock tunnels. Several layers of extremely tough doors were designed to protect against a direct hit. Although the converters are fairly efficient, feeding up to 10 MVA with 90% conversion efficiency or so, several hundreds of kilowatts of heat has to be cooled away. This explains a lot about the shelter design that seems strange at first sight. A schematic description of a complete installation can be seen on this drawing from 1951 at Gunnar Ekeving's excellent Swedish rail site. Q26 denotes the transformer for incoming grid 130 kV / 50 Hz three-phase power, Q38 is a rotary converter and Q39 switching equipment + outgoing 15 kV / 16 2/3 Hz single-phase transformer.
These installations became highly classified and were extensively protected and camouflaged. Their number reached around fifty at its peak, but the WWII-doctrines eventually became obsolete with modern warfare and probably more importantly – the rail network’s importance for the military diminished over time. Around 1990, these installations were gradually phased out and scrapped, where most of the equipment has been taken out. Some of them have been sealed with concrete where others are just "casually closed", requiring no violence nor damage to get in and have a look. Enter and leave without a trace is my trademark...
These installations were always located adjacent to a rail line to allow +100 ton converter beasts to be moved in and out by rail. Almost all of these sites are located out in the middle of nowhere so the push for restoring the land is close to zero, leaving the sites empty and abandoned "as is". Anyone with an idea what to do with a stripped bunker could probably make a bargain by asking the rail authority for a deal. A creative team has actually converted one of these abandoned installations into a paintball hall.
A nerdish subject for a gallery ? Maybe, but wait a second to judge until you stand there yourself, forcing up the rusty side door with a loud squeak. Sweeping around the beam from your torch into a pitch dark damp tunnel with rusty remains on the walls... Just wonder what future generations would think about these remains. An overgrown secret garage... Whatfor... ?