The way a railgun works is by creating a (hopefully) controlled, focused, high-powered short-circuit across the rails via the charge.

Here's a quick look at what's happening to answer your question: From what I can tell, it looks like he's got a hotrail-type railgun, meaning there's not an intermediary on/off switch for the rail current. That's ok, because the charge itself works as a switch (more on that in a moment). Note that the rail currents will be polar opposites (+/-).

This next bit's important: When the charge is introduced into the chamber, it MUST have momentum (this is the reason for the compressed air to push it in there). What happens if it doesn't have positive momentum? The charge will actually weld itself to the rail. This is a catastrophic event (system-wise), and the rails must be swapped out before being useful again (preps to OP for the forethought to make these modular).

OK, now it's time to stick a fork in the electric socket: (except this thing is running at 1050V, ~9-10x what your house is running if you're in the United States, or ~5x if you're in just about anywhere else) So if all of the aforementioned things have gone well, then the charge (as our perverbial fork) is going to cause a short across the rails as it completes the circuit between them. This releases quite a bit of energy all at once; enough to ignite a plasma in the chamber (witness the rail damage after shots 1 & 2).

Now to answer your question: Here's the part where the charge's forward momentum is important: the charge remains in motion on its own behalf right up until just before it hits the rails. As the short occurs, the charge's momentum gives it just enough of a forward bias on the short that the energy released is added to the existing forward momentum. So beyond the initial completion of the circuit when the charge hits the rails, I believe the rail length is arbitrary. Once the short hits, that charge knows exactly where it's going.

For the nay-sayers: Its worth mentioning so people don't get bent out of shape: yes, there is a magnetic field running along those highly charged rails that some people believe is at least some part responsible for the exit velocity of the charge. Considering the forces involved when the short occurs, and the pressure when the plasma ignites, I don't put a lot of stock in that theory. What it may help with is the charge's ROTATIONAL velocity, which may help stabilize the forward velocity, but that's just icing, not the cake.