Answer
$F = \frac{1.5\times 10^6}{\sqrt{100+1.5~t}}~~~N$
Work Step by Step
We can find an expression for the speed of the train:
$K = K_0+P~t$
$\frac{1}{2}mv^2 = \frac{1}{2}mv_0^2+P~t$
$v^2 = v_0^2+\frac{2~P~t}{m}$
$v = \sqrt{v_0^2+\frac{2~P~t}{m}}$
$v = \sqrt{(10~m/s)^2+\frac{(2)(1.5\times 10^6~W)~t}{2.06\times 10^6~kg}}$
$v = \sqrt{100+1.5~t}~~m/s$
We can find an expression for the force:
$F~v = P$
$F = \frac{P}{v}$
$F = \frac{1.5\times 10^6}{\sqrt{100+1.5~t}}~~~N$
