Gaya Chennai has a separate rake. 12389 departs on Sunday morning and 12398 arrives Gaya at that time and if mahabodhi rake is used then how could mahabodhi depart on Sunday.
The other thing is that Mahabodhi rake is fit for 130 mps whereas that of Gaya Chennai rake is not. So how could even their rake is being changed.
Earlier when Mahabodhi ran at 110 then it used Gaya Chennai ICF rake on Thursday so that LHB rake could get proper maintenance one day.