available for purchase, the funding model for the company, the number of station tokens available, or which types of trains may be purchased by the company. Corporate mergers and demergers – some games feature optional, or forced, mergers or splits of one or more companies. Corporate stock-ownership – some games enable companies to hold their own stock, purchase private corporations, and/or own the stock of other companies (even to the point of owning or as prelude to merger). Train Types – some may offer multiple types of trains with distinct capabilities or lifetimes. Some trains may "degrade" into other train types upon certain events (for example, delayed obsolescence of 4-trains in 18MEX, or normal trains becoming H-trains in 1844). Trains may become available in unusual sequences. For example, in 1830, diesel engines are available as soon as the first 6-train is purchased — all the 6-trains are not required to be purchased first. In 1824, G-train availability is controlled by when normal trains are purchased. Certain trains may be restricted in terms of which locations they may run to or may count revenue from, or they may provide bonuses for running to certain locations. For example, in 1844, H-trains are prohibited from running to off-board locations. In 1854, only Orient Express trains may run to certain off-board areas. In 1889, diesels get special bonuses for off-board locations. In 1826, E-trains and TGVs ignore dot-towns. TGVs in 1826 and 4D-trains in 18MEX double the value of cities they count. In 1824, only G-trains may run to mines and the corporation always gets the value of the mine rather than it being potentially paid to stockholders. Theme – a few titles eschew the common railroad/stock-market theme. For example, 2038 involves space exploration of the asteroid belt, while Ur, 1830 BC involves building dams and canals in ancient Mesopotamia (in the latter game, "corporations", "presidents" and stock "shares" are represented by kingdoms, rulers and parcels of land). Conventions and tournaments A number of conventions have at least some emphasis on 18XX games, including the Chattanooga Rail Gaming Challenge, held in January or February in Chattanooga, Tennessee and run by Mark Derrick. 18XX games also figure prominently in various "RailCon" and "Puffing Billy" tournaments at many conventions. Play by email and Online 18xx games are often played remotely using various tools that support play by email games. Many games in the genre are implemented