In normal trains AC compartments the batteries are self generated for producing AC, so it takes few additional batteries in case of AC(like if 4 batteries are used in sleeper,8 batteries should be used in self generated AC compartment) which results in high load and high pulling force as these are linked to dynamo of compartment tyres, which avoids acceleration. thats the reason we find normal express trains wont have more than 7-8 AC coaches.
In case of Rajdhani,duronto where all compartments are AC self generated 18-20 coaches wont work with the required speed. so they use EOG for current generation and the compartments are of light weigh and have minimal batteries which will serve for light, fans and other minimal things...
more... remaining power will be provided by EOG. This makes them run fast and less motive force on engines ,better acceleration and breaking.