If the bike has vertical dropouts you can get an ENO hub. As you can see in the pic, Enos are designed so that the actual axle is offset slightly from the line that goes between the two axle nuts. Meaning the line between the two nuts becomes a pivot that the axle can revolve around to regulate chain tension. ENOs work great but at $150 or so they are expensive compared to the cheaper track hubs out there, and you might just want to start with a bike with horizontal dropouts.