It is not just SER but all the zones throughout. It is done to give all the coaches equal amount of travelling KMs. Not only the spare coaches but sometimes long and short distance trains coaches are interchanged. SRC coaching, SYAE, ER/Tikiapara Coaching, TATA Coaching and Sealdah Coaching all said the same thing when asked. Well this is just about ER and SER. Do see trains of NR/Delhi and other zones like SCR and SWR you will find the same. Reason - the same. The 2 Humsafar Rakes of 2018 are now fully mixed with 2016 and 2017 made coaches of HWH - YPR and SRC - PUNE HS rakes. When they were used for Pravasi Special and AJNI - PUNE HS inauguration they consisted of only 2018 made coaches but not now.