2010 Track Circuit Question

[greensky52]

Ah, I thought I did a mistake when I got the negative resistance in exam, so I checked for many times and finally did the calculation again, but still got negative.... Because of this, I did not have enough time to answer two long questions..... But I thought although it was a mistake of examiners, it showed that I did not have enough basic knowledge for this. If I was good enough,I should see the error at first glance~
Mod 5 must fail. I will take it next year