Ludd III Railroad

Flavour

 * Real-world trans-Saharan railway
 * Command System from Consider Phlebas
 * Crazy real-world military designs seriously guys what the fuck?
 * ATOMIC LOCOMOTIVES
 * Choo choo all aboard

The Ludd Rail Network
The current rail network is a mix of local Luddite steam trains of various gauges and multi-kilometre long combustion powered locomotives on Mechanicus standard gauge that ship ore (and no small quantity of stow-away migrants and merchants) from outlying mining settlements to Raspurn for processing.

Proposed extension of the network, currently underway, calls for colossal 12m gauge rails to connect all the major settlements and mining communities of the planet. Massive fission powered locomotives are being constructed to tow a range of carriages, including four-storey passenger conveyors, bulk ore transport wagons by the hundred, and mobile construction yards designed to deploy thousands of personnel and megatonnes of equipment anywhere on the planet to rapidly construct entire towns or fortresses in a matter of weeks. Tracks are constructed with power and communication infrastructure buried nearby, lowering the cost of bringing these services to remote settlements. Though the economic benefits offered by such a system are substantial, its primary purpose is defending Ludd III. To this end, the network is designed with many features that aid in its continued functioning even under heavy attack.

Track Design
Firstly, the tracks are poured ferrocrete, flush with the surrounding terrain and with metal rails embedded in small trenches. The sand and dust of the deserts and steppes is allowed to cover the rails, being blown aside by locomotives as they approach, making tracks difficult to spot from the air, particularly when they haven't been used for some time. Being flush with the ground and constructed robustly, the tracks are hard to damage with aerial bombardment, requiring either heavy ordnance or direct hits to put them out of action.

Bunker Network
Second, a network of hardened bunkers, accessible by rail, is scattered across the planet. Some of these are dug deep into the ground, with tunnels tens of kilometres long gradually descending into the bedrock and blast-shielded sidings for the trains to pull into. Others are built into mountains - offshoots of tunnels that bore through entire mountain ranges. Most of these bunkers connect two or more independent tracks, enabling the trains to exit and continue their missions if one entrance is rendered inaccessible. Each bunker is well stocked with supplies, including weapons and digging equipment, and features hardlines to the planet's decentralised communications networks.

Trains of War
Third, the Ludd PDF is extensively trained and well equipped for deploying via rail. Whole brigades can mount up on the troop and materiel transport wagons and relocate anywhere on the planet relatively quickly with less logistical challenge and expense compared to dropships. The prodigious size of the railway allows for even the heaviest materiel in the Highdown arsenal to be moved around with ease; each cargo wagon can carry four Macharius Heavy Tanks complete with crew and ammunition, or twice that number of Leman Russ tanks. Fire support is also provided by rail, with military trains (and even many civilian ones) featuring one or more fire platform cars. These fearsome war wagons bristle with battle-, (flak capable) earthshaker-, and AA-mounted gatling cannon.

Ground-to-Orbit Torpedoes
Fourth, while the Ludd system does maintain orbital planetary defence systems, its most potent weapon can be found riding the rails across the surface of Ludd III. In a feat of engineering hubris, the Ludd railway is designed to operate as a distributed anti-voidship weapons system. Hundreds of erector-launcher carriages circle the Ludd rails, hidden in amongst regular transport trains. Each one carries a conventional voidship-fired torpedo with melta warhead, modified with a solid fuel first stage to take it out of the atmosphere, using a cold launch system. Much of the mass of the carriage is taken up with water tanks, which are dumped out by sprayers immediately before launch to prevent the sonic shock of the solid fuel booster igniting from blowing the train to smithereens; during normal operations the tanks serve to shield the melta warhead's background emissions from radiation scanners, reducing risk of detection.

Command and Control
Finally, and perhaps most critically to the long-term defence of Ludd, the rail network provides continuity of Command and Control structure in all but the most devastating attack. Three C&C trains are to be constructed, each powered with a more expensive but correspondingly more effective plasma drive and void shielded to the same standards as Imperial Titans. They are difficult to distinguish from Ludd's larger fleet of war trains, being comparably well armed and armoured, though their main purpose is to act as mobile command centres for the PDF and their performance in terms of speed and survivability substantially outstrips the lesser trains. Each is outfitted with the sort of communications suites more typically seen on the bridge of a capital ship and forests of transmission and receiving gear to ensure that they are always connected. The command system is mirrored across the three vehicles so that one of the other two can take over should the current active command centre be disabled or cut off from the others. In very high threat situations they can retreat to the depths of the rail bunker system, where they can continue coordinating the planet's defence from the safety afforded by kilometres of metamorphic rock.