15 Trains

Key:
M = Married Pair (Motor[A] - Motor[B])
S = Single (Trailer[S])

5 Cars:
(a) M - S - M

7 Cars:
(a) M - S - M - M
(b) M - M - S - M

8 Cars:
(a) M - S - M - S - M

9 Cars:
(a) M - S - M - M - M
(b) M - M - S - M - M
(c) M - M - M - S - M

10 Cars:
(a) M - S - M - M - S - M
(b) M - S - M - S - M - M
(c) M - M - S - M - S - M

11 Cars:
(a) M - S - M - M - M - M
(b) M - M - S - M - M - M
(c) M - M - M - S - M - M
(d) M - M - M - M - S - M

(S3) M - S - M - S - M - S - M