Railguns consist of two parallel metal rails, between which a projectile held in an armature is loaded, completing a circuit between them. A massive electrical current of the order of one million amperes is applied, creating an electromagnetic field, which in turn produces a force that accelerates the projectile along the rails.

The flame is associated with the launching case which is called a sabot. The sabot is made of aluminium and shorts out between the 2x 'rails' accelerating the projectile with 33-megajoule of electrical energy stored in a warehouse sized capacitor bank.

The 1 million amps causes arcing, which vaporises the surface of the rails. The friction between the projectile and the rails also generates extreme heating of the sabot.

Pause to consider that a single megajoule is roughly equivalent to a 1-ton car travelling at 100 mph. The kinetic energy a railgun can deliver could do as much damage as a Tomahawk missile,

http://www.naval-technology.com/features/featureus-navy-electromagnetic-railgun-programme/