For those wondering why we are commenting on dropout (AKA: Track End) length is that when you change gears, your wheel moves up and back depending on your chainring/cog combination.
Longer dropouts allow for this. Shorter dropouts may actually require you to use 2 different chains (with 1 chain section removed) to achieve a normal range of gear options. Changing the chain at the same time as you change chainring and cog is a pain in the ass.
Here are some long droputs:
PlanetX:
Tiemeyer:
Dolan Forza:
Older Dolan DF3:
And the grandaddy of 'em all: BT
All BT frames use the same dropouts.