Underrail 2: Infusion

Underrail is a post-apocalyptic role-playing game focused on combat and exploration. It features turn-based combat, character customization, and an item crafting system. The player controls a single character, whose development and interactions with the game world are the focus of the gameplay. The game's ruleset is mainly inspired by the SPECIAL system from the Fallout series.

It is a classless system with multiple levels of customization. Base ability scores determine a character's core potential, skills represent a character's linear progression in specific skills and feats can grant new abilities, provide passive bonuses or alter a character's existing abilities. The turn-based combat system is similar to Fallout's with more options added on top of it, such as the use of special abilities, psionics and more combat utilities. The system is intended to provide many combat options and unique playstyles.

The game is set in the far future when the Earth's surface has been uninhabitable for a long time and the remaining humans live in the eponymous Underrail metro network, where conflicting factions struggle violently to survive in the harsh underground conditions. The scarcest resources in Underrail are food and living space. There is also a greater background plot concerning the nature of time, but the game is mostly about the inhabitants of Underrail, their politics, strife and the dangers of Underrail itself.

The player assumes the role of a new citizen of South Gate Station (SGS), one of the larger settlements in the South Underrail region. Early in the game, the protagonist is tasked by station councillor Hadrian Tanner with assisting in repair efforts following a massive earthquake that has disrupted train routes and isolated the station. These tasks culminate in a mission to Depot A, a hazardous, mutant-infested junkyard, to retrieve a circuit board needed to repair drilling equipment.

Game engine development for Underrail began in late 2008.

The game's custom engine and development tools are built using C#, Microsoft's .NET 4.0 framework and XNA Game Studio 4.0. The game has been in full-time development since late 2009 and it was known as Timelapse Vertigo during its pre-alpha and earlier development stages. Prior to its early access release on Windows, most of the development work including design, programming, writing and graphics excluding character models and title screen, was done by Dejan Radisic. Since its initial demo version release in 2012, updated versions of Underrail were released roughly every three or four months.