The complete report goes something like this:
(PNBE-HWH), Detained at MKA(Mokama) station for smoke emission from Coach no. 02153/CZAC/ECR due to PP side Alternator pully jammed. ACCI cut all V belts and load transfer to NPP side and worked normal. Time- 06.57-07.29(32-2=30" extra). Primary-RNCC/ECR.
It seems that the ACCI (AC coach in charge) didn't have the requisite tools to cut the V-belt with him, as things like these aren't very common. so they had to wait until a major station came up where they would find the necessary tools for the operations....
more...

until then, the train had to be hauled without compromising safety of the coaches and the passengers inside.
an ac coach has something called as RMPU or roof mounted package unit, which do the work of providing conditioned air inside coaches (AKA air conditioners). these are rated at 25 kw each, and there are two per coach. these are powered by the alternators i mentioned, and in the absence of alternator's pulley rotating, whatever power that ideally should be generated by the motion of the V belt was now instead getting dissipated as heat on the stationary pulley. this energy is quite high, and hence the LP had to keep on inspecting the under carriage to see if any flames have erupted at the concerned spot. the train could be driven very slowly to avoid the problem, but apart from blocking the section it would have also lead to discharging of batteries provided underneath the coach. these batteries supply power when train is stationary or too slow, as in that case alternator can't generate enough power on its own. too slow and the batteries would get discharged by a huge amount, too fast and the coach would have probably caught fire owing to immensely high heat generated, so he took the middle path and drove at around 60 :P
I don't exactly remember the speed, but i believe that below 40 Kmph, alternators don't supply power for AC, rather its the battery which does the job, so he was driving above that speed.