37 Trains

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

5 Cars:
(a) T - M
(b) M - T

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

8 Cars:
(a) T - T - M
(b) T - M - T
(c) M - T - T

9 Cars:
(a) T - M - M - M
(b) M - T - M - M
(c) M - M - T - M
(d) M - M - M - T

10 Cars:
(a) T - T - M - M
(b) T - M - T - M
(c) T - M - M - T
(d) M - T - M - T
(e) M - M - T - T
(f) M - T - T - M

11 Cars:
(1-a) T - M - M - M - M
(1-b) M - T - M - M - M
(1-c) M - M - T - M - M
(1-d) M - M - M - T - M
(1-e) M - M - M - M - T

(3-a) T - T - T - M
(3-b) T - T - M - T
(3-c) T - M - T - T
(3-d) M - T - T - T

12 Cars:
(a) T - T - M - M - M
(b) T - M - T - M - M
(c) T - M - M - T - M
(d) T - M - M - M - T
(e) M - T - M - M - T
(f) M - M - T - M - T
(g) M - M - M - T - T
(h) M - T - T - M - M
(i) M - T - M - T - M
(j) M - M - T - T - M