One EOG is sufficient to power a full-length mixed LHB rake. The capacity is not an issue. The main problems are -
1) Backup - Every EOG coach has 2 diesel gensets, with one supposed to be used as a backup. When a train runs with 2 EOGs, usually one diesel genset from each EOG is used, while the other is left as a backup.
When one EOG is removed, while the rake can still work without any issues, the backup is lost now.
2)...
more... Shunting and connecting/disconnecting coaches - When the through coaches are being removed/added, more steps will be involved because of the additional electrical connections to be made. This makes the process more cumbersome, and will need personnel with some training.
No.2 is a problem for every train with through coaches.
No.1 is not an issue for trains like Kalka mail which have sectional coaches that do not go anywhere after being separated from the main train (separated and stopped at CDG) and the remaining sectional coaches only have a short journey left (CDG-Kalka). So, a backup is not really needed for the final leg of the journey.
However, for trains like Paschim, which have through coaches that have a relatively long journey after bifurcation (Ambala-Kalka, Ambala-Amritsar) running with one EOG and no backup may not be desirable .