
Answer:
 L = 6944.44 m
Explanation:
For this exercise, let's look for the current consumed by each train drop
     P = I V
     I = P / V
     I = 6000/25000
     I = 0.24 A
This is the current consumed by each train down.
Let's calculate the current that carries the cables
     V = I R
     I₂ = V / R
     I₂ = 25000/15
     I₂ = 1666.67 A
As the resistance value increases linearly with the length of the cable, let's use a rule of proportions, to find the total length
     L = 1666.67 (1 / 0.24)
     L = 6944.44 m
This is the length that a downturn can travel before the current is insufficient to move it