Wiring methods for service conductors must be one of the 19 enumerated in Section 230.43 of the 2014 NEC. Interestingly, this list does not include cable trays. This would appear to mean they are excluded. But are they implied because cable bus is in that list?
The answer to this question is no. Cable bus (Art. 370) and Cable Tray (Art. 392) are two very different wiring methods. Unfortunately, there’s nothing in 230.43 to tell you to keep reading. If you do, you find the answer