The efforts of Nimbasa City to develop a safe and functional subway system were stymied throughout its construction by the local underground's large population of Excadrill. These pokemon occasionally appear on the surface, but they travel primarily by digging and build their nests deep underground. For most of the city's history, this was not a problem, for their network of tunnels carried them far below the foundations of the city's architecture, and the Excadrill soon learned the hard way that although it was easy to drill through concrete, human retaliations made it extremely inadvisable.
The same consideration could not be given to the subway's construction, which stretched deep beneath the city, walling off large portions of their habitat and often appropriating existing Excadrill tunnels for trains. The Excadrill remained a constant nuisance after the project's completion, drilling not only through the tracks and walls (which were swiftly repaired and extensively maintained to prevent tragic accidents) but even the trains themselves.
To combat the Excadrill, Nimbasa City sent for Unova's top trainers to defend the passengers of these trains, and, intrigued by both the challenge and the rare items given in payment, they eagerly accepted. At first, their pokemon rode atop the train and tried to engage these Excadrill as they came, but the Excadrill were so enthralled by the challenge that instead of trying to derail trains, they boarded at every stop to ensure themselves longer, more exciting battles.
With this new threat, the train cars where these trainers rode were reinforced from the inside, in order to withstand the chaos of a pokemon battle. The trainers, renowned already for their skill, tested their abilities against each other as well as the Excadrill, who stopped attacking trains in favor of fighting on them, and the Battle Subway was born.
