Thankfully, Unova's underground is less like the London one where you're often pressed against walls by big sweaty men, then someone keeps prodding you with an umbrella and then you spend half your time getting lost, so we're thankful for that. The Battle Subway is somewhere in between a mere Battle Tower and a full Battle Frontier. However to call it a Battle Frontier would be too generous but it is a unique facility which is bound to provide lengthy battles and worthwhile prizes.

The Battle Subway has 8 lines, 7 are with specific battle circumstances and all of those require you to beat seven trainers in a row for Battle points. This works exactly like it does in the games before it, the more trainers you battle, the higher your streak, the more battle points you receive and the harder the battles become. It's worth noting all battles bring your Pokemon's level to 50 regardless if they are over or under it.

However, there's one noticeable difference in the Battle Subway from the usual Battle Tower/Frontiers and such is that the three Battle Modes (Single, Double, Multi) have a Normal Line and a 'Super' Line. But to gain access to the Super line you must gain a 21 battle streak on the Normal corresponding line and face the relevant Subway master (as shown down below.) Its worth noting, in the normal lines, only Pokemon from Unova will be used, while in the Super variations you'll be fighting a much wide range of Pokemon.
After you've won 21 win streak in the Normal line you won't go further in that line and will restart from battle one, the Super lines will still be open though.