Primary Maintenance happens at the base Station where the train is homed, Secondary Maintenance happens after a train travels a certain distance which upon crossing that threshold, the Train would require Secondary Maintenance, For Primary Maintenance, a Rake requires 8 hrs of lieover time at the station of which 6 is for the actual maintenance and around 2hrs is for shunting purposes, im unsure of SM Period

in Primary Maintenance, everything about a rake is checked and errors are rectified if any. In Secondary Maintenance this is also same. There is also...
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another type of maintenance called "OEM or Other End Maintenance" which is done for trains which would return from its destination the same day usually on the Platform itself, it contains watering of train and rake cleaning.

Primary maintenance is done at the base CDO, i.e., the station to which the rake belongs. It may or may not be one of the originating stations. It is the complete inspection and maintenance for the rake's next run. The coaches and it's equipments are checked for any defects, and underslung equipment is checked and repaired if needed.

Primary maintenance also consists of inspection of the brakes, hydraulics, AC components, and oil changes if needed. Wheel alignments are also checked. Scheduled biotoilet tank cleanup, pest management is done during PM. The coaches...
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are also washed and cleaned from inside and outside. 6 hours are allotted for primary maintenance.

Secondary maintenance depends on the train. While premium trains such as Rajdhani, Duronto and Shatabdi undergo SM after every run, regular trains require SM after 4000 km round-trip if LHB and 3500 km if ICF.

It consists of checking the trains running equipment, brakes and interior components, water filling, cleaning the coach as per requirements. It's not as thorough as PM, but checks the train's fitness for the next run to it's home station.

Some trains get platform return, due to time constraints or unavailable maintenance facilities, so they're cleaned and water filling is done on the platform itself before the next run.

RBPC or Round Trip Brake Power Certificate is issued by the base CDO to trains not undergoing SM at the other end of journey. Again premium trains are exceptions and are given BPC at both end of journey.

Short distance regular trains require maintenance or BPC when they complete 4000km- LHB/3500km- ICF, even if they touch the base station multiple times during this distance.