Let T1 be the pull on the last car of the train
I tried to solve this problem by calculate the acceleration first. I used this equation F=ma a=F/M = (1000N)/(6000kg+850kg+1700kg)
And then I used T1-f = ma T
1-680n = 850kg x a from the above answer

But I didn't get the answer. Can anyone point out my mistake? thx so much!