Once upon a time, there was a great king called Vikramaditya - the WAM-4 - Shashank

The ride indices actually indicate how long a person would be able to comfortably ride in that coach for a standard acceleration/deceleration pattern. Normal stand-alone coaches are used in express trains and passengers, so naturally their frequency of acceleration and deceleration is lesser than an EMU. And also, EMU's invariably have higher accelerations compared to a loco hauled train.
Thus, with EMU's being targeted specifically for short distance, frequent halt journeys, the ride index need not be really high. A ride index of 4 indicates that, with a worst case pattern of accelerating and braking, and for a given track condition, an EMU can be comfortable for a 1.5-2 hour journey. And note that the ride index...
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improves with improvement in track standards and also with a change in halt patterns.
Almost all ICF EMU's come with pneumatic suspension, which already guarantees a better ride quality at high speeds and high loads. The ride index though, in this case, is severely affected by the frequent halts and also slightly due to the amount of cost cutting that goes into building these.