How do you mean designated alphabetically from the front? Do you mean like people are situated according to their surnames? I am quite sure that is not the case. The cars are booked out from the centre to the front and then to the back (I think). Seats are numbered with the higher number at the front end of the carriage and seat number one at the rear of the carriage.
I mean the car numbers A, B, C and so on are numbered from the front of the set? I'm in car E on one trip and car C on the return.
I'm in seat 12 both times, so just interested in the car seat numbering plan.