About homing and switches...

[Kitwn]

JW,
Not sure on the 'index' thing but even closed loop servos do not indicate absolute position so you do need homing switches. Whereas stepper motors merely assume each step pulse sent to the driver represents a real movement at the motor, the feedback sensors on servo motors produce pulses confirming that a certain amount of movement really has taken place. For both types of motor your controller must have a known starting position from which to count pulses and calculate current position.
It is possible to have multi-section sensor discs which can tell the absolute rotational position of the motor/disc but that still doesn't tell you where you are along the axis. The only way to do that is to fit a linear optical track (the best ones are apparently made of glass) alongside the rails and sense absolute position from that. A combination of linear track and rotating disc can provide absolute position with high accuracy but not within my pocket-money budget!

Kit
PS Andy beat me to an answer while I was typing. The type of servo motors I'm thinking of have an optical disc attached to provide a train of pulses as the shaft rotates rather than a potentiometer. Counting pulses tells the controller how far the shaft has turned.