The accident occurred soon after the train left the Dulhazara station in Chakaria Upazila
A pistol, bomb-making materials and a 'large' number of jihadist books were seized from them.
The bus from Manikganj was headed for Dhaka's Gabtoli when the elite police unit stopped it for a routine search around 9:30pm, RAB-4 Nabinagar camp commander Maj Abdul Hakim said.
The details will be revealed later after questioning the suspects, he added.