I went to Dictionary.com to search for the meaning "trainset"
and this is the reply:
Similarly, the term trainset refers to a group of rolling stock that is permanently
or semi-permanently coupled together to form a unified set of
equipment (the term is most often applied to
passenger train configurations).
http://www.reference.com/search?r=13&q=Trainset
I also went to search for C751B in Wikipedia:
21 trainsets of 6 cars
each were purchased
http://en.wikipedia.org/wiki/Kawasaki_Heavy_Industries_%26_Nippon_Sharyo_C751B_cars
Furthermore, taken from his own post...
Q2.
View picture here(Too big):
http://srv0106-10.oak1.imeem.com/g/05135e0eb61e1c2ed620aac439b65329.jpg
What trainset is this?
A: 001/002
B: 059/060
C: 085/086
D: 111/112
By the meaning "trainset" as stated above it means a singular
term, i.e. the number "1", so isn't it obvious he also stated that
6 cars that were coupled together means ONE trainset?

Think very clear liao la... there are only 21 trainsets of C751Bs
consisting of 6 cars per trainset. 