Section 28(6) in The Railways (Opening for Public Carriage of Passengers) Rules, 2000
(6)A rolling stock having different principal dimension or a different bogie design or new designs of braking system or suspension details like axle load, track loading density, unsprung mass being different shall be regarded as new rolling stock:Provided that any minor change of equipment design or change of internal equipment layout on the rolling stock or minor change of axle load or minor change of track loading density or minor change in unsprung mass shall not be regarded as new rolling stock, unless such changes are likely to significantly affect weight distribution, center of gravity or riding behaviour of the rolling stock:Provided further that any modification in the design of the coach which alters the system of operation and control over the rolling stock, like change in the braking system or change in the principle of traction shall be regarded as a material modification in the type and design of the rolling stock.
