"One of their light walkers carried a weapon of lethal effect. It fired a form of ultra-high velocity projectile. I saw one of our tanks after having been hit by it. There was a small hole punched in either flank - one the projectile's entry point, the other its exit. The tiny munition had passed through the vehicle with such speed that everything within the hull not welded down had been sucked out through the exit hole. Including the crew. We never identified their bodies, for all that remained of them was a red stain upon the ground extending some twenty metres from the wreck."
- — Major Kane, Mordian 607th Heavy Armoured Regiment
Rail Weapons are large Tau ballistic weapons that make use of electromagnetically-induced linear acceleration to fire a projectile at hypersonic speeds. Rail Weapons typically possess an extreme range, and are particularly renowned for their destructive capabilities upon single targets, especially against armoured vehicles or fortifications.
Rail Weapons use a series of powerful electromagnets and super-conductive electrodes to accelerate small, solid and conductive projectiles to hypersonic speeds. The resulting high levels of kinetic force imparted to the weapon's projectile allows it to devastate and destroy the most heavily armoured vehicles on impact, even at extremely long ranges. Few vehicles are able to withstand a direct hit from a Rail Weapon without suffering critical damage. Because the projectile is capable of achieving a velocity of anywhere between six and ten times the speed of sound, it generates a tremendous force when it strikes its target that is often far more than would be produced by an explosive charge of the same size. As such, the speed of the projectile is enough to punch through virtually any amount of armour. The size of the weapon determines the strength of its power source and thus the kinetic force imparted to the type of projectile used as ammunition.
Rail Weapons usually fire single, solid projectiles and rely on the projectile's speed to penetrate armour, even at long ranges. Because the projectiles are so small, large numbers can be carried by the weapon's operator at a time. However, when used against infantry, a Railgun can have a devastating effect in which the impact of the round will simply vaporise the target. Those near the travelling path of the projectile will also literally find the breath sucked out of their lungs, such is its speed. A Railgun makes an iconic "whip-crack" sound when it fires, which is created when the Railgun's round breaks the sound barrier. Many enemy soldiers have learned to dread its distinct noise, which is only heard after the round has already hit home.
The basic physical principle behind Rail Weapon technology is well known to the Adeptus Mechanicus, but the Imperium has never been able to utilise it in a worthwhile form due to the myriad of issues inherent in the operation of such weapons. The limitations of such technology include the staggering amount of power required to propel the projectile along the rail, and the dissipation of the heat generated by the process. The Tau appear to have implemented solutions to these problems, and many Hereteks amongst the Adeptus Mechanicus would very much like to understand how this was accomplished.
Theorised Railgun Mechanics
A Rail Weapon functions through the manipulation of the electromagnetic force. A magnetically responsive projectile in an electrically conductive sheath is inserted between two conductive rails attached to a power source, making a complete circuit. As the current moves through the circuit it generates a magnetic force that pushes outward, forcing the projectile down the rails. The further the projectile goes out, the greater the current that goes through and the stronger the magnetic force becomes, causing the projectile to accelerate along the full length of the rails, achieving a tremendous velocity by the time it passes the end of the rails and breaks the circuit. After this, another round can be readied as soon as enough charge can be built back up for the next shot.
To achieve the charge necessary to fire a shell at combat velocity typically requires a bulky array of capacitors attached to a powerful generator, which tends to limit the use of most battlefield Rail Weapons to dedicated vehicles. While a Rail Weapon is conceptually simple, manufacturing and fielding one presents several engineering challenges, requiring highly conductive materials with strong resistance to thermal expansion, efficient cooling systems, very precise machining processes, and methods of quickly servicing and re-machining the rails in the field to keep their interior surfaces in pristine condition as even minor wear could have a significantly deleterious affect on the weapon's performance.
Known Rail Weapons
- Rail Rifle - A potent weapon, the Rail Rifle makes use of the same technologies as the larger Railgun and Heavy Rail Rifle, albeit on a smaller scale so that it is infantry portable. Possessed of an impressively long range and ability to deal great damage, the Rail Rifle's true strength lies in its armour penetration capabilities. The Rail Rifle's hypersonic projectile is able to punch through even Terminator Armour with ease. As a dedicated elite infantry killer, enemy units that manage to survive a Rail Rifle fusillade often find themselves pinned down, unable to do anything in the face of such deadly fire. The Rail Rifle also proves effective when used against light vehicles. Rail Rifles have only recently been authorised for issue to frontline Tau units, having completed an extensive field-testing phase.
- Heavy Rail Rifle - A Heavy Rail Rifle is similar to the larger Railgun, but is slightly smaller so that it can be mounted on a Tau Battlesuit. Whilst this weapon’s range and damage is not as great as its larger cousin, it is still an impressive anti-tank weapon, renowned for its effectiveness at penetrating enemy armour at extreme ranges. The XV88 Broadside Battlesuit is the only Tau unit that is armed with the Heavy Rail Rifle, and the XV88 makes use of it as a twin-linked primary weapons system.
- Railgun - Railguns are renowned for their capability to penetrate enemy armour at extreme ranges. The Railgun is most commonly mounted as a primary weapon on a Hammerhead gunship. Along with standard solid-shot rounds, a Railgun is also able to fire sophisticated submunition rounds, which consist of smaller projectiles that can strike a wide area. Whilst the overall damage imparted on impact with this variant of the Railgun's projectile is less than the standard shot, such submunitions are extremely useful for suppressive area fire against infantry targets.
- Heavy Railgun - A Heavy Railgun is a larger version of the standard Railgun and is usually only mounted on the Manta and AX-1-0 variant of the Tiger Shark aircraft. Heavy Railguns fire larger rounds at higher speeds, and these projectiles are fin-stabilised to provide extra lift, and therefore greater range when fired in-atmosphere.
- Starship Railgun Batteries - Despite recent Tau advances in superconductor technology, Railguns mounted on Tau starships are of such enormous size and require such large amounts of energy to fire that they are grouped into batteries and fired one barrel at a time in sequential order, ensuring that the first barrel is loaded and ready before it is charged again. Tau starship Railgun batteries are considered comparable in firepower and effectiveness to the weapons batteries of Macrocannons employed by Imperial starships.
- Battlefleet Gothic Armada (Specialty Game Background Book), pg. 98
- Chapter Approved 2004, pg. 46
- Codex: Tau (3rd Edition), pg. 17
- Codex: Tau Empire (4th Edition), pg. 30
- Codex: Tau Empire (6th Edition), pp. 42, 50, 66
- Deathwatch: Mark of the Xenos (RPG), pg. 6
- Imperial Armour Volume Three - The Taros Campaign, pp. 68, 175, 198, 208, 284
- Rogue Trader: Tau Character Guide (RPG), pg. 28
- Warhammer 40,000: Munitorum - Rail Rifles (Digital Edition)