Originally Posted by T-Mar

It not just track bicycles. Pre-derailleur road racing bicycles were the same and were often marketed as road/track bicycles. The rear fork end slot length had to accommodate flip-flop hubs with a small fixed gear cog used on the flats and a larger, freewheel cog used on major climbs and descents. Campagnolo's Cambio Corsa derailleurs also used wheel position, rather than chain tensioners, to deal with different cog sizes and the proprietary dropouts had very long slots.