It all boils down to what happens when a single ball hits an identical ball - if there is no energy loss, the parting speed will be the same as the speed of approach - they will 'exchange velocities. Total momentum and energy remain the same. You can extend this to a whole row of balls.

We did the sums in sixth form A level mechanics and it all came out with some very simple calculations.

You can end up with an increase in speed when you have different masses and with both objects moving towards each other - e.g firing a ball bearing at the front of an express locomotive, when the ball bearing will bounce back with (almost) the sum of the two initial speeds.