Buy at BNC or SBC before boarding for there isnt much on board. At BWT, you'll find some Dosas and Idlies but they taste below avg and from BWT onwards its only Samosas and Samosas -at JTJ, KPD, CTO everywhere its the Samosa vendors inside and outside the coaches. And yes, pls avoid them.
Chai coffee valas also make their presence felt but not a much as the Samosa guys.